Dark/bright: Auto (Android's dark theme), Dark or Bright, kept across restarts

Settings has a segmented Auto / Dark / Bright. Auto follows platformBrightness, so
Android's schedule too. The Dash's quick button holds the other variant; a long press
is auto again. The mode is kept with shared_preferences.

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

lib/theme/palette.dart

@@ -20,6 +20,7 @@ import 'dart:math' show max, min;
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.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.
@@ -116,29 +117,70 @@ class Palette {
   );
 }
 
-/// A theme: both variants, and which one is showing. Listen to it to follow a switch.
-class Themes extends ChangeNotifier {
-  Themes(this.name, this.dark, this.bright);
+/// Which variant shows: Android's own dark theme (auto), 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.
+class Themes extends ChangeNotifier with WidgetsBindingObserver {
+  Themes(this.name, this.dark, this.bright, [this._prefs]) {
+    _mode =
+        VariantMode.values.asNameMap()[_prefs?.getString(_key)] ??
+        VariantMode.auto;
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  static const _key = 'variant_mode';
 
   final String name;
   final Palette dark;
   final Palette bright;
-  bool _isDark = true;
+  final SharedPreferences? _prefs;
+  late VariantMode _mode;
 
-  bool get isDark => _isDark;
-  Palette get current => _isDark ? dark : bright;
+  VariantMode get mode => _mode;
 
-  set isDark(bool v) {
-    if (v == _isDark) return;
-    _isDark = v;
+  set mode(VariantMode v) {
+    if (v == _mode) return;
+    _mode = v;
+    _prefs?.setString(_key, v.name);
     notifyListeners();
   }
 
-  void toggle() => isDark = !_isDark;
+  bool get isDark => switch (_mode) {
+    VariantMode.auto =>
+      WidgetsBinding.instance.platformDispatcher.platformBrightness ==
+          Brightness.dark,
+    VariantMode.dark => true,
+    VariantMode.bright => false,
+  };
+
+  Palette get current => isDark ? dark : bright;
+
+  /// The quick switch: hold the other variant fixed.
+  void toggle() => mode = isDark ? VariantMode.bright : VariantMode.dark;
+
+  @override
+  void didChangePlatformBrightness() {
+    if (_mode == VariantMode.auto) notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
 
   /// [themeName], falling back to `default`, then to the hard-coded palette
   /// (smart-serow's chain).
   static Future<Themes> load([String name = themeName]) async {
+    SharedPreferences? prefs;
+    try {
+      prefs = await SharedPreferences.getInstance();
+    } catch (_) {
+      // No storage: the mode starts at auto each time.
+    }
     for (final n in {name, 'default'}) {
       try {
         final j = jsonDecode(
@@ -148,19 +190,25 @@ class Themes extends ChangeNotifier {
           n,
           Palette._fromJson(j['dark'] as Map<String, dynamic>),
           Palette._fromJson(j['bright'] as Map<String, dynamic>),
+          prefs,
         );
       } catch (_) {
         // Missing or malformed: try the next one.
       }
     }
-    return Themes('fallback', Palette.fallbackDark, Palette.fallbackBright);
+    return Themes(
+      'fallback',
+      Palette.fallbackDark,
+      Palette.fallbackBright,
+      prefs,
+    );
   }
 
   /// Material's theme for the rest of the app, from the same roles: buttons and
   /// selection in accent, errors in alarm, raised surfaces in surface.
   ThemeData material() {
     final p = current;
-    final brightness = _isDark ? Brightness.dark : Brightness.light;
+    final brightness = isDark ? Brightness.dark : Brightness.light;
     return ThemeData(
       useMaterial3: true,
       brightness: brightness,