feat: 增加AI配置文件与TTS自动朗读，修复二级页面返回按钮与音频播放

kouyu_english/lib/core/app_state.dart

@@ -9,6 +9,8 @@ import 'a0_core.dart';
 import 'generated_content.dart';
 import 'local_store.dart';
 import 'seed_courses.dart';
+import 'ai_config.dart';
+import 'ai_service.dart';
 
 class AppState extends ChangeNotifier {
   static const _storageKey = 'learning_state_v1';
@@ -262,6 +264,22 @@ class AppState extends ChangeNotifier {
 
   Future<void> load() async {
     try {
+      final config = await AiConfigFile.loadFromAsset();
+      if (config != null) {
+        if (config.apiKey != null && config.apiKey!.trim().isNotEmpty) {
+          AiService.instance.setFallbackApiKey(config.apiKey);
+        }
+        if (aiEndpoint.isEmpty && config.endpoint.isNotEmpty) {
+          aiEndpoint = config.endpoint;
+        }
+        if (aiModel.isEmpty && config.model.isNotEmpty) {
+          aiModel = config.model;
+        }
+        if (aiProvider == AiProviderType.mock &&
+            config.provider != AiProviderType.mock) {
+          aiProvider = config.provider;
+        }
+      }
       String? raw;
       if (!_usesLegacyTestStore) {
         raw = await LocalSnapshotStore.instance.read();
@@ -275,6 +293,19 @@ class AppState extends ChangeNotifier {
         }
       }
       if (raw != null) _restore(jsonDecode(raw) as Map<String, dynamic>);
+      if (config != null) {
+        if (aiEndpoint.isEmpty && config.endpoint.isNotEmpty) {
+          aiEndpoint = config.endpoint;
+        }
+        if (aiModel.isEmpty && config.model.isNotEmpty) {
+          aiModel = config.model;
+        }
+        if (aiProvider == AiProviderType.mock &&
+            config.provider != AiProviderType.mock &&
+            raw == null) {
+          aiProvider = config.provider;
+        }
+      }
     } catch (_) {
       // A corrupt local cache must never prevent access to offline lessons.
     } finally {
@@ -283,6 +314,19 @@ class AppState extends ChangeNotifier {
     }
   }
 
+  Future<bool> reloadAiConfigFromAsset() async {
+    final config = await AiConfigFile.loadFromAsset();
+    if (config == null) return false;
+    aiProvider = config.provider;
+    aiEndpoint = config.endpoint;
+    aiModel = config.model;
+    if (config.apiKey != null && config.apiKey!.trim().isNotEmpty) {
+      AiService.instance.setFallbackApiKey(config.apiKey);
+    }
+    notifyListeners();
+    return true;
+  }
+
   void _restore(Map<String, dynamic> data) {
     onboardingComplete =
         data['onboardingComplete'] as bool? ?? onboardingComplete;