Auto dark/bright takes a light sensor where the device allows one

lib/theme/light.dart: dark at 15 lux or less, bright from 60, held 5 s; Settings shows the
reading. The Redmi Pad SE refuses apps its only light sensor (Xiaomi's, from the front
camera), so there Auto keeps following Android's dark theme. DASH-NEXT.md records the
other lux sources.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/theme/palette.dart

@@ -15,6 +15,7 @@
 //
 // Pick the theme at build time: `--dart-define=YUKART_THEME=default` (default: rei, smart-serow's).
 
+import 'dart:async';
 import 'dart:convert';
 import 'dart:math' show max, min;
 
@@ -22,6 +23,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
+import 'light.dart';
+
 /// The HUD face (smart-serow's DIN 1451). Opt-in: a widget names it in its own
 /// TextStyle; everything else keeps the basic font, which covers CJK where DIN does not.
 const hudFont = 'DIN1451';
@@ -117,12 +120,12 @@ class Palette {
   );
 }
 
-/// Which variant shows: Android's own dark theme (auto), or one held fixed.
+/// Which variant shows: auto (the light sensor, else Android's dark theme), or one held fixed.
 enum VariantMode { auto, dark, bright }
 
-/// A theme: both variants, and which one is showing. Listen to it to follow a switch,
-/// and, in auto, Android's dark theme turning on or off (by hand or on its schedule).
-/// The mode is kept across restarts.
+/// A theme: both variants, and which one is showing. Listen to it to follow a switch.
+/// In auto it follows the tablet's light sensor ([followLight]); without one, Android's
+/// dark theme (by hand or on its schedule). The mode is kept across restarts.
 class Themes extends ChangeNotifier with WidgetsBindingObserver {
   Themes(this.name, this.dark, this.bright, [this._prefs]) {
     _mode =
@@ -148,10 +151,50 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
     notifyListeners();
   }
 
+  final _light = LightSwitch();
+  Timer? _holdTimer;
+
+  /// The last light level, in lux, while a sensor reports (for Settings, to tune by).
+  final lux = ValueNotifier<double?>(null);
+
+  /// Follow a light sensor in auto. Errors (no sensor) leave auto on Android's dark theme.
+  void followLight(Stream<double> levels) {
+    void changed(bool? before) {
+      if (_light.isDark != before && _mode == VariantMode.auto) {
+        notifyListeners();
+      }
+    }
+
+    void feed(double l) {
+      final before = _light.isDark;
+      _light.feed(l, DateTime.now());
+      changed(before);
+      // A sensor may report only on change: finish a hold even if none comes.
+      _holdTimer?.cancel();
+      if (_light.pending) {
+        _holdTimer = Timer(LightSwitch.hold, () {
+          final before = _light.isDark;
+          _light.settle();
+          changed(before);
+        });
+      }
+    }
+
+    levels.listen(
+      (l) {
+        lux.value = l;
+        feed(l);
+      },
+      onError: (_) => lux.value = null,
+      cancelOnError: true,
+    );
+  }
+
   bool get isDark => switch (_mode) {
     VariantMode.auto =>
-      WidgetsBinding.instance.platformDispatcher.platformBrightness ==
-          Brightness.dark,
+      _light.isDark ??
+          WidgetsBinding.instance.platformDispatcher.platformBrightness ==
+              Brightness.dark,
     VariantMode.dark => true,
     VariantMode.bright => false,
   };
@@ -163,12 +206,14 @@ class Themes extends ChangeNotifier with WidgetsBindingObserver {
 
   @override
   void didChangePlatformBrightness() {
-    if (_mode == VariantMode.auto) notifyListeners();
+    if (_mode == VariantMode.auto && _light.isDark == null) notifyListeners();
   }
 
   @override
   void dispose() {
     WidgetsBinding.instance.removeObserver(this);
+    _holdTimer?.cancel();
+    lux.dispose();
     super.dispose();
   }