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/features/onboarding/onboarding_pages.dart

@@ -70,10 +70,16 @@ class _WelcomePageState extends State<WelcomePage> {
 }
 
 class PlacementPage extends StatefulWidget {
-  const PlacementPage({super.key, required this.state, required this.onStart});
+  const PlacementPage({
+    super.key,
+    required this.state,
+    required this.onStart,
+    this.onBack,
+  });
 
   final AppState state;
   final VoidCallback onStart;
+  final VoidCallback? onBack;
 
   @override
   State<PlacementPage> createState() => _PlacementPageState();
@@ -96,6 +102,16 @@ class _PlacementPageState extends State<PlacementPage> {
       PlacementLevel.simpleConversation: ('能简单对话', '想说得更自然、更有信心'),
     };
     return AppPage(
+      appBar: widget.onBack != null
+          ? AppBar(
+              leading: IconButton(
+                icon: const Icon(Icons.arrow_back),
+                tooltip: "返回",
+                onPressed: widget.onBack,
+              ),
+              title: const Text("基础定位"),
+            )
+          : null,
       child: SpacedColumn(
         spacing: 14,
         children: [