优化录制页面的相机预览逻辑，增加预览未就绪时的错误提示，确保用户体验更流畅。

lib/features/recording/pages/page_record.dart

@@ -135,7 +135,13 @@ class _RecordingPageState extends ConsumerState<RecordingPage> {
     final result = await ref
         .read(recordingViewModelProvider.notifier)
         .ensureCameraAndMicrophonePermissions();
-    if (result.allGranted) return true;
+    if (result.allGranted) {
+      final ready = ref.read(recordingViewModelProvider).session.isPreviewReady;
+      if (ready) return true;
+      if (!mounted) return false;
+      AppToast.show('相机预览启动失败，请重试');
+      return false;
+    }
     if (!mounted) return false;
 
     await RecordDialog.showSingle(