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, 12 insertions, 2 deletions

diff --git a/player-android/lib/main.dart b/player-android/lib/main.dart
index 2c1b193..3358383 100644
--- a/player-android/lib/main.dart
+++ b/player-android/lib/main.dart
@@ -64,6 +64,16 @@ void main() async {
 }
 
 // ---------------------------------------------------------------------------
+// Module-level theme constants
+// ---------------------------------------------------------------------------
+
+// Built once at startup rather than on every rebuild of PlayerAndroidApp.
+// ThemeData construction is not cheap, and the colour tokens never change
+// at runtime — only the active ThemeMode (light/dark/system) does.
+final _lightTheme = buildLightTheme();
+final _darkTheme = buildDarkTheme();
+
+// ---------------------------------------------------------------------------
 // Root widget
 // ---------------------------------------------------------------------------
 
@@ -92,8 +102,8 @@ class PlayerAndroidApp extends ConsumerWidget {
       title: 'Player',
       routerConfig: router,
       // Material 3 is enabled in both ThemeData instances; see theme_provider.dart.
-      theme: buildLightTheme(),
-      darkTheme: buildDarkTheme(),
+      theme: _lightTheme,
+      darkTheme: _darkTheme,
       themeMode: themeMode,
     );
   }