fix: rebuild media stream when switching camera

[BackGwa]

Description

Return and set a new MediaStream instance on camera switch so React and peer connections detect the stream change reliably. This prevents stale stream references from interrupting video track replacement.

Type of Change

• Bug fix (bugs/)
• New feature (features/)
• Refactoring (refactor/)
• Hotfix (hotfix/)
• Chore (chore/)

Related Issues

Stream interruption during screen transitions #31

Changes Made

• mediaService.switchCamera now constructs and returns a new MediaStream instance (new video track + existing audio tracks) instead of mutating the current stream in place, so the stream reference changes on every switch
• useMediaStream.switchCamera now calls setStream(newStream, 'camera') with the returned stream to update React context, propagating the new reference through the component tree
• The changed reference causes the usePeerConnection effect to re-run and invoke sender.replaceTrack(track) on all active peer connections, delivering the new camera feed to participants without dropping the WebRTC connection

Checklist

• Code follows the project's coding style
• Self-review completed