feat(auth): 登录页改为 ymb 品牌并支持后台配置背景图

lib/app.dart

@@ -14,6 +14,7 @@ import 'core/data/remote/catalog_api.dart';
 import 'core/data/remote/category_api.dart';
 import 'core/data/remote/comment_api.dart';
 import 'core/data/remote/commerce_api.dart';
+import 'core/data/remote/config_api.dart';
 import 'core/data/remote/device_profile.dart';
 import 'core/data/remote/download_api.dart';
 import 'core/data/remote/interaction_api.dart';
@@ -35,6 +36,7 @@ import 'core/data/repositories/playlist_repository.dart';
 import 'core/data/repositories/search_repository.dart';
 import 'core/data/repositories/store_repository.dart';
 import 'core/data/repositories/watch_history_repository.dart';
+import 'core/data/services/app_config_controller.dart';
 import 'core/data/services/auth_controller.dart';
 import 'core/data/services/auth_session_store.dart';
 import 'core/data/services/catalog_controller.dart';
@@ -208,6 +210,11 @@ class _LumaDramaAppState extends State<LumaDramaApp> {
 
   late final LocaleController _localeController = LocaleController();
 
+  /// 后台下发的启动配置（登录页背景等）；拉不到就用本地默认。
+  late final AppConfigController _appConfigController = AppConfigController(
+    ConfigApi(_apiClient),
+  );
+
   /// 登录期内共享的「已解锁收费集」：播放页进出、feed 与全屏切换都要沿用。
   late final UnlockedEpisodeRegistry _unlockedEpisodes =
       UnlockedEpisodeRegistry();
@@ -218,6 +225,7 @@ class _LumaDramaAppState extends State<LumaDramaApp> {
     // 恢复本地令牌与设备号；未登录也能正常浏览，不阻塞首屏
     _auth.load();
     unawaited(_localeController.load());
+    unawaited(_appConfigController.load());
     // 登录/登出时同步收藏状态：收藏属于账号，不能跨账号串联
     _auth.addListener(_onAuthChanged);
     unawaited(_favoriteController.load());
@@ -252,6 +260,7 @@ class _LumaDramaAppState extends State<LumaDramaApp> {
     _walletController.dispose();
     _interactionController.dispose();
     _localeController.dispose();
+    _appConfigController.dispose();
     _apiClient.close();
     super.dispose();
   }
@@ -304,6 +313,7 @@ class _LumaDramaAppState extends State<LumaDramaApp> {
         ChangeNotifierProvider.value(value: _walletController),
         ChangeNotifierProvider.value(value: _interactionController),
         ChangeNotifierProvider.value(value: _localeController),
+        ChangeNotifierProvider.value(value: _appConfigController),
         ChangeNotifierProvider.value(value: _auth),
       ],
       child: Consumer<LocaleController>(