feat(client): 主题选择 controller + SharedPreferences 持久化

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/core/theme/theme_controller.dart

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+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const _kKey = 'app_theme';
+const _kValid = {'a', 'b', 'c'};
+
+class ThemeController extends StateNotifier<String> {
+  ThemeController() : super('a');
+
+  Future<void> restore() async {
+    final prefs = await SharedPreferences.getInstance();
+    final v = prefs.getString(_kKey);
+    if (v != null && _kValid.contains(v)) state = v;
+  }
+
+  Future<void> set(String key) async {
+    if (!_kValid.contains(key)) return;
+    state = key;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_kKey, key);
+  }
+}
+
+final themeControllerProvider =
+    StateNotifierProvider<ThemeController, String>((ref) => ThemeController());

client/test/theme/theme_controller_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:jiu_client/core/theme/theme_controller.dart';
+
+void main() {
+  setUp(() => SharedPreferences.setMockInitialValues({}));
+
+  test('默认 a；set 持久化并可 restore', () async {
+    final c = ProviderContainer();
+    expect(c.read(themeControllerProvider), 'a');
+    await c.read(themeControllerProvider.notifier).set('c');
+    expect(c.read(themeControllerProvider), 'c');
+
+    final c2 = ProviderContainer();
+    await c2.read(themeControllerProvider.notifier).restore();
+    expect(c2.read(themeControllerProvider), 'c');
+  });
+
+  test('非法 key 被忽略', () async {
+    final c = ProviderContainer();
+    await c.read(themeControllerProvider.notifier).set('zzz');
+    expect(c.read(themeControllerProvider), 'a');
+  });
+}