Fix Material 3 theming review issues (task fb)

- Roll back ThemeNotifier state on SharedPreferences write failure so
  in-memory and disk never diverge
- Fix stale comment: scaffoldBackgroundColor ← --bg-body (not background)
- Fix source attribution: theme.css not docs/theming.md
- Make _ThemeToggle const-constructible; remove incorrect ignore comment
- Inline _buildSegmentedButton into _ThemeToggle.build (was a trivial passthrough)
- Add defensive isNotEmpty comment on SegmentedButton.onSelectionChanged
- Hoist buildLightTheme()/buildDarkTheme() to module-level finals in main.dart
  so ThemeData is built once at startup rather than on every rebuild
- Add _ThemeToggle tests: initial segment selection and segment tap dispatch

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 110 insertions, 0 deletions

diff --git a/player-android/test/screens/settings_screen_test.dart b/player-android/test/screens/settings_screen_test.dart
index 71c2d7d..ecbc94d 100644
--- a/player-android/test/screens/settings_screen_test.dart
+++ b/player-android/test/screens/settings_screen_test.dart
@@ -21,6 +21,7 @@ import 'package:player_android/api/dio_client.dart';
 import 'package:player_android/providers/api_client_provider.dart';
 import 'package:player_android/providers/auth_state_provider.dart';
 import 'package:player_android/providers/settings_provider.dart';
+import 'package:player_android/providers/theme_provider.dart';
 import 'package:player_android/screens/settings_screen.dart';
 
 // ---------------------------------------------------------------------------
@@ -65,6 +66,25 @@ class _FakeSettingsNotifier extends SettingsNotifier {
   }
 }
 
+/// In-memory [ThemeNotifier] that bypasses [SharedPreferences] in tests.
+///
+/// Starts at [ThemeMode.system] (the default) and records the last mode passed
+/// to [setThemeMode] so tests can assert on the captured value.
+class _FakeThemeNotifier extends ThemeNotifier {
+  // Tracks the last mode applied via setThemeMode for test assertions.
+  ThemeMode? capturedMode;
+
+  @override
+  Future<ThemeMode> build() async => ThemeMode.system;
+
+  @override
+  Future<void> setThemeMode(ThemeMode mode) async {
+    capturedMode = mode;
+    // Mirror the production behaviour: update in-memory state immediately.
+    state = AsyncData(mode);
+  }
+}
+
 // ---------------------------------------------------------------------------
 // Helper: pump SettingsScreen inside a minimal ProviderScope.
 // ---------------------------------------------------------------------------
@@ -260,6 +280,96 @@ void main() {
   });
 
   // --------------------------------------------------------------------------
+  // Theme toggle
+  // --------------------------------------------------------------------------
+
+  group('theme toggle', () {
+    testWidgets('renders with system segment selected by default',
+        (tester) async {
+      final fakeStorage = _FakeTokenStorage().._token = 'alice';
+      final fakeSettings = _FakeSettingsNotifier('http://10.0.2.2:8080');
+      final fakeTheme = _FakeThemeNotifier();
+
+      final router = GoRouter(
+        initialLocation: '/',
+        routes: [
+          GoRoute(
+            path: '/',
+            builder: (_, __) => const SettingsScreen(),
+          ),
+          GoRoute(
+            path: '/login',
+            builder: (_, __) => const Scaffold(body: Text('Login')),
+          ),
+        ],
+      );
+
+      await tester.pumpWidget(
+        ProviderScope(
+          overrides: [
+            tokenStorageProvider.overrideWithValue(fakeStorage),
+            settingsProvider.overrideWith(() => fakeSettings),
+            themeProvider.overrideWith(() => fakeTheme),
+          ],
+          child: MaterialApp.router(routerConfig: router),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      // The segmented button should be present with the system segment selected.
+      final button = tester.widget<SegmentedButton<ThemeMode>>(
+        find.byKey(const Key('settings_theme_toggle')),
+      );
+      expect(button.selected, equals({ThemeMode.system}));
+    });
+
+    testWidgets('tapping a segment calls setThemeMode with the right ThemeMode',
+        (tester) async {
+      final fakeStorage = _FakeTokenStorage().._token = 'alice';
+      final fakeSettings = _FakeSettingsNotifier('http://10.0.2.2:8080');
+      final fakeTheme = _FakeThemeNotifier();
+
+      final router = GoRouter(
+        initialLocation: '/',
+        routes: [
+          GoRoute(
+            path: '/',
+            builder: (_, __) => const SettingsScreen(),
+          ),
+          GoRoute(
+            path: '/login',
+            builder: (_, __) => const Scaffold(body: Text('Login')),
+          ),
+        ],
+      );
+
+      await tester.pumpWidget(
+        ProviderScope(
+          overrides: [
+            tokenStorageProvider.overrideWithValue(fakeStorage),
+            settingsProvider.overrideWith(() => fakeSettings),
+            themeProvider.overrideWith(() => fakeTheme),
+          ],
+          child: MaterialApp.router(routerConfig: router),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      // Scroll the segmented button into view before tapping — the settings
+      // screen content may exceed the test viewport height.
+      await tester.ensureVisible(find.byKey(const Key('settings_theme_toggle')));
+      await tester.pumpAndSettle();
+
+      // Tap the "Dark" segment label.
+      await tester.tap(find.text('Dark'));
+      await tester.pumpAndSettle();
+
+      // The fake notifier should have received ThemeMode.dark.
+      expect(fakeTheme.capturedMode, equals(ThemeMode.dark));
+    });
+  });
+
+  // --------------------------------------------------------------------------
   // Logout flow
   // --------------------------------------------------------------------------