Updated Videochat GUI

Refactored `LivekitVideoManager.cs`:
- Changed `NetworkBehaviour` to `MonoBehaviour`
- Removed `RepublishVideo` method
- Updated camera methods:
  - Last selected camera saved in `PlayerPrefs`
  - Unpublished video if camera changed
  - Camera activates when video is published
  - Camera deactivates when video is unpublished
- Added `StatusPanel` for publish status

Modified `SEENewWorld`:
- Removed `LivekitVideoManager` object

Updated `SettingsMenu` prefab:
- Centered exit button
- Added `VideochatStatusPanel` as first child:
  - Added `RawImage` for status image (assigned to `LivekitVideoManager` script)
  - Added `Text` for status display (assigned to `LivekitVideoManager` script)
- Added `VideochatPanel` as child:
  - Added `Dropdown` (assigned to `LivekitVideoManager` script)