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,
+);