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/ai_config.dart

@@ -0,0 +1,74 @@
+import 'dart:convert';
+import 'package:flutter/services.dart';
+import 'models.dart';
+
+class AiConfigFile {
+  const AiConfigFile({
+    required this.provider,
+    required this.endpoint,
+    required this.model,
+    this.apiKey,
+    this.description,
+  });
+
+  final AiProviderType provider;
+  final String endpoint;
+  final String model;
+  final String? apiKey;
+  final String? description;
+
+  static const String defaultAssetPath = 'assets/config/ai_config.json';
+
+  factory AiConfigFile.fromJson(Map<String, dynamic> json) {
+    final providerStr = json['provider'] as String? ?? 'compatible';
+    final provider = AiProviderType.values.firstWhere(
+      (p) => p.name.toLowerCase() == providerStr.toLowerCase(),
+      orElse: () => AiProviderType.compatible,
+    );
+    return AiConfigFile(
+      provider: provider,
+      endpoint: (json['endpoint'] as String? ?? '').trim(),
+      model: (json['model'] as String? ?? '').trim(),
+      apiKey: json['apiKey'] as String?,
+      description: json['description'] as String?,
+    );
+  }
+
+  factory AiConfigFile.parse(String rawJson) {
+    final data = jsonDecode(rawJson) as Map<String, dynamic>;
+    return AiConfigFile.fromJson(data);
+  }
+
+  Map<String, dynamic> toJson() => {
+    'provider': provider.name,
+    'endpoint': endpoint,
+    'model': model,
+    if (apiKey != null) 'apiKey': apiKey,
+    if (description != null) 'description': description,
+  };
+
+  static Future<AiConfigFile?> loadFromAsset([
+    String path = defaultAssetPath,
+  ]) async {
+    try {
+      final content = await rootBundle.loadString(path);
+      return AiConfigFile.parse(content);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  AiConfigFile copyWith({
+    AiProviderType? provider,
+    String? endpoint,
+    String? model,
+    String? apiKey,
+    String? description,
+  }) => AiConfigFile(
+    provider: provider ?? this.provider,
+    endpoint: endpoint ?? this.endpoint,
+    model: model ?? this.model,
+    apiKey: apiKey ?? this.apiKey,
+    description: description ?? this.description,
+  );
+}