Fix interrupted video play during screen capture

[HocKaTop]

This fixes a screen capture startup issue where video.play() can be interrupted before the media element is ready.

The previous flow assigned srcObject and immediately called play(). In some browsers this can reject with:

The play() request was interrupted because the media was removed from the document

Changes:

• wait for loadedmetadata before calling video.play()
• prevent duplicate capture startup with a starting guard
• clean up the stream if startup fails
• pause and detach the video source during stop

Tested locally with:

npm install
npm run dev