feat(flutter/theme): themeModeProvider with secure-storage persistence

AsyncNotifier wrapping flutter_secure_storage's "theme_mode" key.
Returns AppThemeMode.system as the default. set(mode) writes the
value + updates state.

Used by MinstrelApp (next commit) to drive MaterialApp.themeMode and
by the Settings appearance section to persist the operator's pick.

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

flutter_client/lib/theme/theme_mode_provider.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../auth/auth_provider.dart';
+
+const _kThemeModeKey = 'theme_mode';
+
+enum AppThemeMode { system, dark, light }
+
+extension AppThemeModeMaterial on AppThemeMode {
+  ThemeMode get materialMode => switch (this) {
+        AppThemeMode.system => ThemeMode.system,
+        AppThemeMode.dark => ThemeMode.dark,
+        AppThemeMode.light => ThemeMode.light,
+      };
+}
+
+class ThemeModeController extends AsyncNotifier<AppThemeMode> {
+  @override
+  Future<AppThemeMode> build() async {
+    final storage = ref.watch(secureStorageProvider);
+    final raw = await storage.read(key: _kThemeModeKey);
+    return switch (raw) {
+      'dark' => AppThemeMode.dark,
+      'light' => AppThemeMode.light,
+      _ => AppThemeMode.system,
+    };
+  }
+
+  Future<void> set(AppThemeMode mode) async {
+    final storage = ref.read(secureStorageProvider);
+    await storage.write(key: _kThemeModeKey, value: mode.name);
+    state = AsyncData(mode);
+  }
+}
+
+final themeModeProvider =
+    AsyncNotifierProvider<ThemeModeController, AppThemeMode>(
+  ThemeModeController.new,
+);

flutter_client/test/theme/theme_mode_provider_test.dart

@@ -0,0 +1,76 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mocktail/mocktail.dart';
+
+import 'package:minstrel/auth/auth_provider.dart';
+import 'package:minstrel/theme/theme_mode_provider.dart';
+
+class _MockStorage extends Mock implements FlutterSecureStorage {}
+
+void main() {
+  late _MockStorage storage;
+
+  setUpAll(() {
+    registerFallbackValue('');
+  });
+
+  setUp(() {
+    storage = _MockStorage();
+    when(() => storage.write(key: any(named: 'key'), value: any(named: 'value')))
+        .thenAnswer((_) async {});
+  });
+
+  test('defaults to system when nothing stored', () async {
+    when(() => storage.read(key: 'theme_mode')).thenAnswer((_) async => null);
+    final container = ProviderContainer(overrides: [
+      secureStorageProvider.overrideWithValue(storage),
+    ]);
+    addTearDown(container.dispose);
+
+    final mode = await container.read(themeModeProvider.future);
+    expect(mode, AppThemeMode.system);
+  });
+
+  test('reads stored "dark" → AppThemeMode.dark', () async {
+    when(() => storage.read(key: 'theme_mode')).thenAnswer((_) async => 'dark');
+    final container = ProviderContainer(overrides: [
+      secureStorageProvider.overrideWithValue(storage),
+    ]);
+    addTearDown(container.dispose);
+
+    final mode = await container.read(themeModeProvider.future);
+    expect(mode, AppThemeMode.dark);
+  });
+
+  test('reads stored "light" → AppThemeMode.light', () async {
+    when(() => storage.read(key: 'theme_mode')).thenAnswer((_) async => 'light');
+    final container = ProviderContainer(overrides: [
+      secureStorageProvider.overrideWithValue(storage),
+    ]);
+    addTearDown(container.dispose);
+
+    final mode = await container.read(themeModeProvider.future);
+    expect(mode, AppThemeMode.light);
+  });
+
+  test('set(.light) writes "light" to storage and updates state', () async {
+    when(() => storage.read(key: 'theme_mode')).thenAnswer((_) async => null);
+    final container = ProviderContainer(overrides: [
+      secureStorageProvider.overrideWithValue(storage),
+    ]);
+    addTearDown(container.dispose);
+
+    await container.read(themeModeProvider.future);
+    await container.read(themeModeProvider.notifier).set(AppThemeMode.light);
+
+    verify(() => storage.write(key: 'theme_mode', value: 'light')).called(1);
+    expect(container.read(themeModeProvider).value, AppThemeMode.light);
+  });
+
+  test('materialMode extension maps each enum to ThemeMode', () {
+    expect(AppThemeMode.system.materialMode.name, 'system');
+    expect(AppThemeMode.dark.materialMode.name, 'dark');
+    expect(AppThemeMode.light.materialMode.name, 'light');
+  });
+}