Stall + adaptive timeout logic:
- Replaced simple fixed upload timeout approach with:
- Stall detection (stalled (no progress for 60s)).
- File-size-based timeout calculation for slow connections.
- Applied consistently across create/upload/finalize request flow.

Firefox camera/webcam recording fix
- Fixed MIME selection so screen and webcam streams can use stream-appropriate MIME types.
- Webcam now uses a MIME type derived from the webcam stream (instead of inheriting screen stream MIME).
- Webcam upload content type is now derived from webcamBlob.type, preventing incorrect codec/content-type propagation.

Edited manifest.json and updated README.md for sendrec-firefox-extension source.