Headless / CLI mode (no GUI)

[gluckdev]

Summary

Add a command-line mode so CastToTV can be scripted / run on a headless box (no Tk window), e.g.:

casttotv --cast "https://youtube.com/watch?v=…" --to 192.168.1.238
casttotv --discover
casttotv --radio "https://dnbradio.com/?channel=darkstep" --to 192.168.1.30
casttotv --cast movie.mkv --to "Living Room" --multi

Scope

• Argument parsing that reuses the existing pieces: fast_discover, YoutubeStreamer / RadioStreamer, build source → play_on, cast_to_all for --multi.
• --to accepts an IP, a friendly-name substring, or all.
• --discover prints the tagged device list and exits.
• Keep it import-light so it works without Tk (the GUI stays the default when no args are given).
• Useful for cron / home-automation and for testing without the GUI.

Acceptance

• --discover lists devices with protocol tags.
• --cast <url|file> --to <target> plays end-to-end with no GUI.
• --radio and --multi supported.