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/main.dart

@@ -55,9 +55,18 @@ class _KouyuEnglishAppState extends State<KouyuEnglishApp> {
               onContinue: () => setState(() => onboardingStep = 1),
             );
           }
-          return PlacementPage(
-            state: appState,
-            onStart: () => appState.finishOnboarding(),
+          return PopScope(
+            canPop: false,
+            onPopInvokedWithResult: (didPop, _) {
+              if (!didPop) {
+                setState(() => onboardingStep = 0);
+              }
+            },
+            child: PlacementPage(
+              state: appState,
+              onBack: () => setState(() => onboardingStep = 0),
+              onStart: () => appState.finishOnboarding(),
+            ),
           );
         },
       ),