feat(app): 剧场列表增加语种筛选

lib/pages/discover/discover_logic.dart

@@ -36,6 +36,7 @@ class DiscoverLogic extends ChangeNotifier {
   Duration searchDebounce = const Duration(milliseconds: 350);
   static const int searchPageSize = 20;
   List<Drama> searchResults = const [];
+  String? selectedLanguage;
   int searchTotal = 0;
   bool searchLoading = false;
   String? searchError;
@@ -87,8 +88,32 @@ class DiscoverLogic extends ChangeNotifier {
 
   /// 搜索态展示后端结果，非搜索态展示剧场自身的可见短剧。
   List<Drama> visibleDramas(List<Drama> fallback) {
-    if (!hasSearchQuery) return fallback;
-    return searchResults;
+    if (!hasSearchQuery) return _filterByLanguage(fallback);
+    return _filterByLanguage(searchResults);
+  }
+
+  /// 当前 feed 中出现过的语种，按首次出现顺序去重。
+  List<String> languagesOf(List<Drama> dramas) {
+    final languages = <String>[];
+    for (final drama in dramas) {
+      final language = drama.language;
+      if (language.isNotEmpty && !languages.contains(language)) {
+        languages.add(language);
+      }
+    }
+    return languages;
+  }
+
+  /// 按语种筛选；null 表示全部。
+  void selectLanguage(String? code) {
+    selectedLanguage = code;
+    notify();
+  }
+
+  List<Drama> _filterByLanguage(List<Drama> dramas) {
+    final code = selectedLanguage;
+    if (code == null) return dramas;
+    return dramas.where((drama) => drama.language == code).toList();
   }
 
   Future<void> retrySearch() async {