feat(flutter): MinstrelApp watches themeMode + offers darkTheme

MaterialApp now provides both light and dark themes plus a
themeMode bound to themeModeProvider. ThemeMode.system rebuilds
the tree automatically when the OS toggles brightness — no manual
listener needed.

While themeModeProvider is loading (storage read in flight), falls
back to AppThemeMode.system, which inherits the OS setting.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/app.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'shared/routing.dart';
 import 'theme/theme_data.dart';
+import 'theme/theme_mode_provider.dart';
 
 class MinstrelApp extends ConsumerWidget {
   const MinstrelApp({super.key});
@@ -10,9 +11,12 @@ class MinstrelApp extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final router = ref.watch(routerProvider);
+    final mode = ref.watch(themeModeProvider).value ?? AppThemeMode.system;
     return MaterialApp.router(
       title: 'Minstrel',
-      theme: buildThemeData(),
+      theme: buildLightTheme(),
+      darkTheme: buildDarkTheme(),
+      themeMode: mode.materialMode,
       routerConfig: router,
     );
   }