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>

test/palette_test.dart

@@ -0,0 +1,36 @@
+import 'package:display/theme/palette.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  final dispatcher = TestWidgetsFlutterBinding.instance.platformDispatcher;
+  tearDown(dispatcher.clearPlatformBrightnessTestValue);
+
+  test(
+    'auto follows Android, the quick switch holds one, and it is kept',
+    () async {
+      SharedPreferences.setMockInitialValues({});
+      final prefs = await SharedPreferences.getInstance();
+      Themes make() =>
+          Themes('t', Palette.fallbackDark, Palette.fallbackBright, prefs);
+      final t = make();
+      var notified = 0;
+      t.addListener(() => notified++);
+
+      expect(t.mode, VariantMode.auto);
+      dispatcher.platformBrightnessTestValue = Brightness.light;
+      expect(t.isDark, isFalse);
+      expect(notified, 1); // the binding tells its observers
+
+      t.toggle(); // bright showing: hold dark
+      expect((t.mode, t.isDark), (VariantMode.dark, true));
+      dispatcher.platformBrightnessTestValue = Brightness.dark;
+      dispatcher.platformBrightnessTestValue = Brightness.light;
+      expect(notified, 2); // fixed: Android's changes are ignored
+
+      expect(make().mode, VariantMode.dark); // kept for the next start
+    },
+  );
+}