refactor(data): create layered architecture with core/data/ui packages

- Move constants from lib/config/ to lib/core/constants/
- Move color themes to lib/core/theme/
- Create LocationData domain model in data/models/
- Create lightweight GpsService in data/services/
- Create LocationRepository in data/repositories/ consolidating
  GpsDataManager business logic (GPS stream, staleness, precision)
- Create shared SpeedDisplay widget in ui/shared/
- Defer overlay feature — stubs reserved in ui/features/overlay/

lib/data/models/location_data.dart

@@ -0,0 +1,42 @@
+/// Domain model representing processed GPS data.
+///
+/// Immutable — use [copyWith] to create modified copies.
+class LocationData {
+  final double speed;           // Raw speed in m/s
+  final double heading;         // Raw heading in degrees
+  final String displaySpeed;    // Formatted speed text (with -- logic)
+  final String displayHeading;  // Formatted heading text
+  final bool isSpeedValid;      // True if from recent GPS
+  final bool isHeadingValid;    // True if from recent GPS
+  final DateTime timestamp;     // Timestamp for staleness detection
+
+  const LocationData({
+    required this.speed,
+    required this.heading,
+    required this.displaySpeed,
+    required this.displayHeading,
+    required this.isSpeedValid,
+    required this.isHeadingValid,
+    required this.timestamp,
+  });
+
+  LocationData copyWith({
+    double? speed,
+    double? heading,
+    String? displaySpeed,
+    String? displayHeading,
+    bool? isSpeedValid,
+    bool? isHeadingValid,
+    DateTime? timestamp,
+  }) {
+    return LocationData(
+      speed: speed ?? this.speed,
+      heading: heading ?? this.heading,
+      displaySpeed: displaySpeed ?? this.displaySpeed,
+      displayHeading: displayHeading ?? this.displayHeading,
+      isSpeedValid: isSpeedValid ?? this.isSpeedValid,
+      isHeadingValid: isHeadingValid ?? this.isHeadingValid,
+      timestamp: timestamp ?? this.timestamp,
+    );
+  }
+}

lib/data/repositories/location_repository.dart

@@ -0,0 +1,332 @@
+import 'dart:async';
+import 'dart:developer' as developer;
+import 'package:flutter/widgets.dart';
+import 'package:geolocator/geolocator.dart';
+import '../../core/constants/gps_constants.dart';
+import '../../services/logger.dart';
+import '../models/location_data.dart';
+import '../services/gps_service.dart';
+
+/// Repository that wraps GpsService and applies GPS business logic.
+///
+/// Single source of truth for location data. Handles:
+/// - GPS stream management (start/stop/restart)
+/// - Data processing (staleness, precision switching, validation)
+/// - Lifecycle management (app foreground/background, wake lock)
+class LocationRepository extends WidgetsBindingObserver {
+  StreamSubscription<Position>? _gpsSubscription;
+  final StreamController<LocationData> _dataController =
+      StreamController<LocationData>.broadcast();
+
+  // Use late to defer DateTime.now() — const constructors can't call methods
+  late LocationData _currentData = LocationData(
+    speed: 0.0,
+    heading: -1.0,
+    displaySpeed: '--',
+    displayHeading: 'N/A',
+    isSpeedValid: false,
+    isHeadingValid: false,
+    timestamp: DateTime.now(),
+  );
+
+  Timer? _staleDataTimer;
+  Timer? _backgroundGracePeriodTimer;
+  bool _isDisposed = false;
+
+  // Lifecycle and precision state
+  LocationAccuracy _currentAccuracy = LocationAccuracy.high;
+  bool _hasFirstFix = false;
+  DateTime? _firstFixTime;
+  bool _isAppInForeground = true;
+  bool _isGpsActive = false;
+
+  // Public stream for UI components to subscribe to
+  Stream<LocationData> get locationStream => _dataController.stream;
+
+  // Get current data synchronously
+  LocationData get currentData => _currentData;
+
+  bool get isInitialized => _isInitialized;
+  bool get isGpsActive => _isGpsActive;
+
+  bool _isInitialized = false;
+
+  /// Initialize GPS: check permissions, get first fix, start stream.
+  Future<void> initialize() async {
+    if (_isInitialized) {
+      Logger.warn('Already initialized, skipping...', 'LocationRepository');
+      return;
+    }
+
+    Logger.info('Starting location repository initialization...', 'LocationRepository');
+    WidgetsBinding.instance.addObserver(this);
+
+    // Check GPS permissions and services first
+    final serviceEnabled = await GpsService.isLocationServiceEnabled();
+    Logger.debug('Location service enabled: $serviceEnabled', 'LocationRepository');
+    if (!serviceEnabled) {
+      Logger.error('GPS service disabled', 'LocationRepository');
+      _updateData(_currentData.copyWith(
+        displaySpeed: 'GPS OFF',
+        displayHeading: 'GPS OFF',
+      ));
+      _isInitialized = true;
+      return;
+    }
+
+    final permissionGranted = await GpsService.requestPermissions();
+    Logger.debug('GPS permission granted: $permissionGranted', 'LocationRepository');
+    if (!permissionGranted) {
+      Logger.error('GPS permission denied', 'LocationRepository');
+      _updateData(_currentData.copyWith(
+        displaySpeed: 'NO PERM',
+        displayHeading: 'NO PERM',
+      ));
+      _isInitialized = true;
+      return;
+    }
+
+    try {
+      // Get initial position with long timeout for cold start
+      final initialPosition = await GpsService.getCurrentPosition(
+        accuracy: LocationAccuracy.bestForNavigation,
+        timeLimit: GpsConfig.INITIAL_FIX_TIMEOUT,
+      );
+
+      if (initialPosition == null) {
+        Logger.error('Initial GPS fix failed — null position', 'LocationRepository');
+        _updateData(_currentData.copyWith(
+          displaySpeed: 'GPS ERR',
+          displayHeading: 'GPS ERR',
+        ));
+        _isInitialized = true;
+        return;
+      }
+
+      Logger.info('Initial GPS fix obtained', 'LocationRepository');
+      _hasFirstFix = true;
+      _firstFixTime = DateTime.now();
+      _onPositionUpdate(initialPosition);
+
+      // Start position stream
+      Logger.debug('Starting GPS stream with accuracy: $_currentAccuracy', 'LocationRepository');
+      _startPositionStream(_currentAccuracy);
+
+      _isGpsActive = true;
+      await GpsService.setWakeLock(true);
+      _isInitialized = true;
+    } catch (e, stackTrace) {
+      developer.log('GPS subscription creation failed: $e',
+          error: e, stackTrace: stackTrace, name: 'LocationRepository');
+      _updateData(_currentData.copyWith(
+        displaySpeed: 'GPS ERR',
+        displayHeading: 'GPS ERR',
+      ));
+      _isInitialized = true;
+    }
+
+    Logger.info('Location repository initialized successfully', 'LocationRepository');
+  }
+
+  void _startPositionStream(LocationAccuracy accuracy) {
+    _gpsSubscription = GpsService.createPositionStream(accuracy: accuracy).listen(
+      (Position position) {
+        Logger.debug('GPS callback triggered', 'LocationRepository');
+        _onPositionUpdate(position);
+      },
+      onError: (error) {
+        Logger.error('GPS stream error callback: $error', 'LocationRepository');
+        _onGpsError(error);
+      },
+      onDone: () {
+        Logger.warn('GPS stream done callback - stream ended', 'LocationRepository');
+      },
+      cancelOnError: false,
+    );
+  }
+
+  void _onPositionUpdate(Position position) {
+    Logger.debug('Raw GPS update: speed=${position.speed.toStringAsFixed(2)} m/s, heading=${position.heading.toStringAsFixed(1)}°', 'LocationRepository');
+
+    _scheduleStaleDataTimer();
+
+    // Validate speed
+    final rawSpeed = position.speed;
+    final bool speedValid = rawSpeed.isFinite && rawSpeed >= 0;
+    final double speed = speedValid ? rawSpeed : 0.0;
+
+    // Validate heading
+    final rawHeading = position.heading;
+    final bool headingValid = rawHeading.isFinite &&
+        rawHeading >= GpsConfig.VALID_HEADING_MIN &&
+        rawHeading < GpsConfig.VALID_HEADING_MAX;
+    final double heading = headingValid ? rawHeading : _currentData.heading;
+
+    // Format display
+    final displaySpeed = speed.toStringAsFixed(1);
+    final displayHeading = headingValid
+        ? GpsService.formatHeading(heading)
+        : _currentData.displayHeading;
+
+    Logger.debug('Processed: displaySpeed="$displaySpeed", displayHeading="$displayHeading"', 'LocationRepository');
+
+    final locationData = LocationData(
+      speed: speed,
+      heading: heading,
+      displaySpeed: displaySpeed,
+      displayHeading: displayHeading,
+      isSpeedValid: speedValid,
+      isHeadingValid: headingValid,
+      timestamp: DateTime.now(),
+    );
+
+    _updateData(locationData);
+
+    // Check precision switching after first-fix gate
+    if (_hasFirstFix && _firstFixTime != null) {
+      final timeSinceFirstFix = DateTime.now().difference(_firstFixTime!);
+      if (timeSinceFirstFix >= GpsConfig.PRECISION_SWITCH_FIRST_FIX_DELAY) {
+        _checkPrecisionSwitch(position.speed);
+      }
+    }
+  }
+
+  void _onGpsError(dynamic error) {
+    if (_isDisposed) return;
+    final errorString = error.toString().toLowerCase();
+    if (!errorString.contains('timeout') && !errorString.contains('temporarily')) {
+      _updateData(_currentData.copyWith(
+        displaySpeed: 'GPS ERR',
+        displayHeading: 'GPS ERR',
+        isSpeedValid: false,
+        isHeadingValid: false,
+      ));
+    }
+  }
+
+  void _scheduleStaleDataTimer() {
+    _staleDataTimer?.cancel();
+    _staleDataTimer = Timer(GpsConfig.STALE_DATA_THRESHOLD, _handleStaleDataTimeout);
+  }
+
+  void _handleStaleDataTimeout() {
+    if (_isDisposed) return;
+    final displayText = _currentData.displaySpeed;
+    if (displayText == '--' || displayText.startsWith('GPS') || displayText == 'NO PERM') {
+      return;
+    }
+
+    final headingStillValid = _currentData.isHeadingValid &&
+        _currentData.heading >= GpsConfig.VALID_HEADING_MIN &&
+        _currentData.heading < GpsConfig.VALID_HEADING_MAX;
+    final displayHeading = headingStillValid ? _currentData.displayHeading : '--';
+
+    Logger.warn('No GPS updates within ${GpsConfig.STALE_DATA_THRESHOLD.inSeconds}s - marking data as stale', 'LocationRepository');
+
+    final staleData = _currentData.copyWith(
+      displaySpeed: '--',
+      displayHeading: displayHeading,
+      isSpeedValid: false,
+      isHeadingValid: headingStillValid,
+    );
+
+    _staleDataTimer = null;
+    _updateData(staleData);
+  }
+
+  void _updateData(LocationData newData) {
+    if (_isDisposed) return;
+    _currentData = newData;
+    _dataController.add(newData);
+    Logger.debug('Broadcasting data: ${newData.displaySpeed} ${newData.displayHeading}', 'LocationRepository');
+  }
+
+  /// Check if GPS precision should be switched based on speed.
+  void _checkPrecisionSwitch(double speedMps) {
+    if (_currentAccuracy == LocationAccuracy.high) {
+      if (speedMps < GpsConfig.PRECISION_DOWN_THRESHOLD_MPS) {
+        _switchPrecision(LocationAccuracy.medium);
+      }
+    } else if (_currentAccuracy == LocationAccuracy.medium) {
+      if (speedMps > GpsConfig.PRECISION_UP_THRESHOLD_MPS) {
+        _switchPrecision(LocationAccuracy.high);
+      }
+    }
+  }
+
+  /// Switch GPS precision and restart stream with new accuracy.
+  void _switchPrecision(LocationAccuracy newAccuracy) {
+    if (_currentAccuracy == newAccuracy) return;
+
+    final speedKmh = _currentData.speed * 3.6;
+    Logger.debug('Precision switched to $newAccuracy at ${speedKmh.toStringAsFixed(1)} km/h', 'LocationRepository');
+
+    _currentAccuracy = newAccuracy;
+    _gpsSubscription?.cancel();
+    _startPositionStream(newAccuracy);
+  }
+
+  /// Handle app lifecycle state changes.
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.paused) {
+      _isAppInForeground = false;
+      // No overlay logic here — overlay is deferred
+      _updateWakeLock();
+    } else if (state == AppLifecycleState.resumed) {
+      _isAppInForeground = true;
+      _backgroundGracePeriodTimer?.cancel();
+      _backgroundGracePeriodTimer = null;
+
+      // Restart GPS if it was stopped
+      if (_gpsSubscription == null && _isInitialized) {
+        restartGps();
+      }
+
+      _updateWakeLock();
+    }
+  }
+
+  /// Stop GPS tracking.
+  void stopGps() {
+    _gpsSubscription?.cancel();
+    _gpsSubscription = null;
+    _isGpsActive = false;
+    _staleDataTimer?.cancel();
+    _updateWakeLock();
+    Logger.info('GPS stopped', 'LocationRepository');
+  }
+
+  /// Restart GPS tracking.
+  void restartGps() {
+    Logger.info('Restarting GPS', 'LocationRepository');
+    _startPositionStream(_currentAccuracy);
+    _isGpsActive = true;
+    _updateWakeLock();
+  }
+
+  /// Update wake lock based on GPS state and app visibility.
+  Future<void> _updateWakeLock() async {
+    final shouldKeepAwake = _isGpsActive && _isAppInForeground;
+    await GpsService.setWakeLock(shouldKeepAwake);
+    Logger.debug(
+      'Wake lock ${shouldKeepAwake ? "enabled" : "disabled"} (GPS: $_isGpsActive, foreground: $_isAppInForeground)',
+      'LocationRepository',
+    );
+  }
+
+  void dispose() {
+    _isDisposed = true;
+    Logger.info('Disposing location repository...', 'LocationRepository');
+    WidgetsBinding.instance.removeObserver(this);
+    _backgroundGracePeriodTimer?.cancel();
+    _backgroundGracePeriodTimer = null;
+    _gpsSubscription?.cancel();
+    _gpsSubscription = null;
+    _staleDataTimer?.cancel();
+    _staleDataTimer = null;
+    _dataController.close();
+    GpsService.setWakeLock(false);
+    Logger.info('Location repository disposed', 'LocationRepository');
+  }
+}

lib/data/services/gps_service.dart

@@ -0,0 +1,73 @@
+import 'dart:async';
+import 'package:geolocator/geolocator.dart';
+import 'package:permission_handler/permission_handler.dart';
+import 'package:wakelock_plus/wakelock_plus.dart';
+import '../../core/constants/gps_constants.dart';
+
+/// Lightweight stateless wrapper around Geolocator.
+///
+/// Only handles platform I/O — no business logic.
+class GpsService {
+  static double? _lastValidHeading;
+
+  /// Create a position stream with dynamic accuracy setting.
+  static Stream<Position> createPositionStream({
+    LocationAccuracy accuracy = LocationAccuracy.high,
+  }) {
+    return Geolocator.getPositionStream(
+      locationSettings: LocationSettings(
+        accuracy: accuracy,
+        distanceFilter: 0,
+      ),
+    );
+  }
+
+  static Future<bool> requestPermissions() async {
+    final permission = await Permission.locationWhenInUse.request();
+    return permission.isGranted;
+  }
+
+  static Future<bool> isLocationServiceEnabled() async {
+    return await Geolocator.isLocationServiceEnabled();
+  }
+
+  static Future<Position?> getCurrentPosition({
+    LocationAccuracy accuracy = LocationAccuracy.bestForNavigation,
+    Duration timeLimit = const Duration(seconds: 60),
+  }) async {
+    try {
+      return await Geolocator.getCurrentPosition(
+        locationSettings: LocationSettings(
+          accuracy: accuracy,
+          timeLimit: timeLimit,
+        ),
+      );
+    } catch (_) {
+      return null;
+    }
+  }
+
+  static double getCachedHeading(double heading) {
+    if (heading >= GpsConfig.VALID_HEADING_MIN && heading <= GpsConfig.VALID_HEADING_MAX) {
+      _lastValidHeading = heading;
+      return heading;
+    }
+    return _lastValidHeading ?? 0.0;
+  }
+
+  static String getCompassDirection(double heading) {
+    final cachedHeading = getCachedHeading(heading);
+    const directions = ['N', 'NE', 'E', 'SE', 'S', 'SW', 'W', 'NW'];
+    final index = ((cachedHeading + 22.5) / 45).floor() % 8;
+    return directions[index];
+  }
+
+  static String formatHeading(double heading) {
+    final cachedHeading = getCachedHeading(heading);
+    return '${cachedHeading.round()} ${getCompassDirection(heading)}';
+  }
+
+  static Future<void> setWakeLock(bool enable) async {
+    await WakelockPlus.toggle(enable: enable);
+  }
+}

lib/ui/features/speedometer/speedometer_view_model.dart

@@ -0,0 +1,81 @@
+import 'dart:async';
+import 'package:flutter/widgets.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../../../core/constants/speed_unit.dart';
+import '../../../data/models/location_data.dart';
+import '../../../data/repositories/location_repository.dart';
+
+/// ViewModel for the speedometer UI.
+///
+/// Thin state manager — subscribes to [LocationRepository] and exposes
+/// the current display state to the view. No data-layer logic here.
+class SpeedometerViewModel extends ChangeNotifier {
+  final LocationRepository _repository;
+  final SharedPreferences _prefs;
+
+  SpeedUnit _displayUnit = SpeedUnit.kmh;
+  LocationData _displayData = LocationData(
+    speed: 0.0,
+    heading: -1.0,
+    displaySpeed: '--',
+    displayHeading: 'N/A',
+    isSpeedValid: false,
+    isHeadingValid: false,
+    timestamp: DateTime.now(),
+  );
+
+  LocationData get displayData => _displayData;
+  SpeedUnit get displayUnit => _displayUnit;
+
+  SpeedometerViewModel({
+    required LocationRepository repository,
+    required SharedPreferences prefs,
+  })  : _repository = repository,
+        _prefs = prefs;
+
+  void init() {
+    _repository.locationStream.listen(_onLocationData);
+    _loadSettings();
+  }
+
+  void _onLocationData(LocationData data) {
+    _displayData = data;
+    notifyListeners();
+  }
+
+  Future<void> _loadSettings() async {
+    _displayUnit = _parseSpeedUnit(_prefs.getString('speed_unit') ?? 'km/h');
+    notifyListeners();
+  }
+
+  static SpeedUnit _parseSpeedUnit(String unit) {
+    switch (unit) {
+      case 'mph':
+        return SpeedUnit.mph;
+      case 'kts':
+        return SpeedUnit.knots;
+      default:
+        return SpeedUnit.kmh;
+    }
+  }
+
+  double get displaySpeed {
+    return _displayUnit.convert(_displayData.speed);
+  }
+
+  String get displaySpeedText {
+    return _displayData.displaySpeed;
+  }
+
+  String get displayHeading {
+    return _displayData.displayHeading;
+  }
+
+  bool get isSpeedValid {
+    return _displayData.isSpeedValid;
+  }
+
+  bool get isHeadingValid {
+    return _displayData.isHeadingValid;
+  }
+}

lib/ui/shared/speed_display.dart

@@ -0,0 +1,95 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:provider/provider.dart';
+import '../../../core/constants/overlay_constants.dart';
+import '../../../core/theme/color_themes.dart';
+import '../../providers/settings_provider.dart';
+
+/// Shared speed display widget — integral + decimal parts with
+/// staleness dimming and tap-to-cycle-theme.
+///
+/// Dumb widget — accepts data as parameters so it works in both
+/// the old Provider-based and new ViewModel-based architectures.
+class SpeedDisplay extends StatelessWidget {
+  final double fontSize;
+  final double opacity;
+  final String speedText;
+  final DateTime timestamp;
+
+  const SpeedDisplay({
+    super.key,
+    required this.fontSize,
+    required this.opacity,
+    required this.speedText,
+    required this.timestamp,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Selector<SettingsProvider, ColorTheme>(
+      selector: (_, s) => s.currentTheme,
+      builder: (context, theme, _) {
+        // Staleness — UI-layer concern
+        final age = DateTime.now().difference(timestamp);
+        final showDashes = age >= OverlayConfig.STALENESS_DASH_THRESHOLD;
+        final isDimmed = age >= OverlayConfig.STALENESS_DIM_THRESHOLD;
+        final staleOpacity = isDimmed ? OverlayConfig.STALENESS_DIM_OPACITY : 1.0;
+        final effectiveOpacity = opacity * staleOpacity;
+
+        final displayText = showDashes ? '--' : speedText;
+
+        return GestureDetector(
+          onTap: () {
+            HapticFeedback.lightImpact();
+            context.read<SettingsProvider>().cycleTheme();
+          },
+          child: _buildSpeedText(displayText, theme, fontSize, effectiveOpacity),
+        );
+      },
+    );
+  }
+
+  Widget _buildSpeedText(String speedText, ColorTheme theme, double fontSize, double opacity) {
+    if (speedText == '--') {
+      return Text(
+        speedText,
+        style: TextStyle(
+          fontSize: fontSize,
+          fontWeight: FontWeight.w300,
+          color: theme.speedText.withValues(alpha: opacity),
+          fontFamily: 'DIN1451Alt',
+        ),
+      );
+    }
+
+    final parts = speedText.split('.');
+    final integralPart = parts[0];
+    final decimalPart = parts.length > 1 ? '.${parts[1]}' : '.0';
+
+    return Row(
+      mainAxisAlignment: MainAxisAlignment.center,
+      crossAxisAlignment: CrossAxisAlignment.baseline,
+      textBaseline: TextBaseline.alphabetic,
+      children: [
+        Text(
+          integralPart,
+          style: TextStyle(
+            fontSize: fontSize,
+            fontWeight: FontWeight.w300,
+            color: theme.speedText.withValues(alpha: opacity),
+            fontFamily: 'DIN1451Alt',
+          ),
+        ),
+        Text(
+          decimalPart,
+          style: TextStyle(
+            fontSize: fontSize * 0.5,
+            fontWeight: FontWeight.w300,
+            color: theme.speedTextSub.withValues(alpha: opacity),
+            fontFamily: 'DIN1451Alt',
+          ),
+        ),
+      ],
+    );
+  }
+}