The sleep mode feature from dev (PR #999) prevented video from auto-starting
when streaming_status == 0, which broke HDMI hotplug functionality.
Modified video_restart_streaming() to check detected_signal:
- No signal + not streaming -> don't start (preserve sleep mode)
- Has signal -> always stop cleanly then restart (fix hotplug)
This ensures proper encoder cleanup and prevents corrupted video output
after HDMI disconnect/reconnect cycles.