feat(viewer): add V4L2 sink output support

Implement the ability to route incoming viewer video streams to a V4L2
loopback device. This allows Raspberry Ninja to function as a virtual
camera source for other applications on the host system.

- Add command-line arguments: `--v4l2sink`, `--v4l2sink-width`,
  `--v4l2sink-height`, `--v4l2sink-fps`, and `--v4l2sink-format`.
- Implement `resolve_v4l2sink_device` to automatically detect or validate
  writable V4L2 output devices.
- Add `_ensure_v4l2sink_chain` and related logic to `publish.py` to
  manage GStreamer pipeline elements for V4L2 output.
- Update `WebRTCClient` to prioritize V4L2 output over standard display
  chains when the feature is enabled.
- Include dependency checks for the `video4linux2` GStreamer plugin.

[auto-enhanced]

Author: steveseguin