Implement camera preview functionality and enhance AssistantPage

- Add a new `useCameraPreview` hook for managing camera access and video stream handling.
- Integrate camera preview capabilities into the AssistantPage, allowing users to toggle video stream visibility.
- Introduce a new toggle for enabling visual understanding, which activates the video stream preview alongside voice functionalities.
- Update the DebugDrawer component to support switching between chat and video views based on the visual understanding setting.
- Refactor related components to accommodate the new camera features and improve user interaction during debugging.

frontend/src/hooks/use-voice-preview.ts

@@ -558,3 +558,5 @@ export function useVoicePreview(
     audioRef,
   };
 }
+
+export type VoicePreview = ReturnType<typeof useVoicePreview>;