Implement `subminer stats -b` to start a background stats daemon and
`subminer stats -s` to stop it, with PID-based process lifecycle
management, single-instance lock bypass for daemon mode, and automatic
reuse of running daemon instances.
- Launcher stats subcommand with cleanup mode
- Stats frontend build integrated into Makefile
- CI workflow updated for stats package
- Config example updated with stats section
- mpv plugin menu entry for stats toggle
