flutter: IMU attitude indicator and UART health check

- WhiskeyMark widget shows roll/pitch as horizon line
- ArduinoData model includes IMU euler angles
- startup waits for Arduino via /health endpoint
- config_service exposes backendUrl

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pi/ui/lib/services/backend_service.dart

@@ -2,14 +2,16 @@ import 'dart:async';
 import 'dart:convert';
 import 'package:http/http.dart' as http;
 
-/// Data from Arduino (voltage, rpm, engine temp, gear)
+/// Data from Arduino (voltage, rpm, engine temp, gear, IMU)
 class ArduinoData {
   final double? voltage;
   final int? rpm;
   final int? engTemp;
   final int? gear; // 0 = neutral, 1-6 = gear
+  final double? roll;  // Euler angle in degrees (negative = left, positive = right)
+  final double? pitch; // Euler angle in degrees (negative = nose down)
 
-  ArduinoData({this.voltage, this.rpm, this.engTemp, this.gear});
+  ArduinoData({this.voltage, this.rpm, this.engTemp, this.gear, this.roll, this.pitch});
 
   factory ArduinoData.fromJson(Map<String, dynamic> json) {
     return ArduinoData(
@@ -17,6 +19,8 @@ class ArduinoData {
       rpm: (json['rpm'] as num?)?.toInt(),
       engTemp: (json['eng_temp'] as num?)?.toInt(),
       gear: (json['gear'] as num?)?.toInt(),
+      roll: (json['roll'] as num?)?.toDouble(),  // IMU mounted with axes swapped
+      pitch: (json['pitch'] as num?)?.toDouble(),
     );
   }
 }

pi/ui/lib/services/config_service.dart

@@ -86,4 +86,11 @@ class ConfigService {
     if (value is String && value.isNotEmpty) return value;
     return _defaultNavigator;
   }
+
+  /// Backend URL for API calls
+  String get backendUrl {
+    final value = _config?['backend_url'];
+    if (value is String && value.isNotEmpty) return value;
+    return 'http://127.0.0.1:5000';
+  }
 }

pi/ui/lib/services/websocket_service.dart

@@ -184,7 +184,10 @@ class WebSocketService {
         final arduino = ArduinoData.fromJson(data);
         _latestArduino = arduino;
         _arduinoController.add(arduino);
-        _log('ard: ${arduino.rpm ?? "-"}rpm ${arduino.voltage ?? "-"}V g${arduino.gear ?? "-"}');
+        final rollStr = arduino.roll != null ? 'r${arduino.roll!.round()}' : '';
+        final pitchStr = arduino.pitch != null ? 'p${arduino.pitch!.round()}' : '';
+        final imuStr = (rollStr.isNotEmpty || pitchStr.isNotEmpty) ? ' $rollStr$pitchStr' : '';
+        _log('ard: ${arduino.rpm ?? "-"}rpm ${arduino.voltage ?? "-"}V g${arduino.gear ?? "-"}$imuStr');
       }
     });