feat: discord presence, cached audio playback, multiinstance safety

Add
- Discord rich presence ("Listening to", cover, progress, Listen Along
  button). enable via [discord] client_id in config.toml : retries quietly
  if Discord isn't running. new dep: discord-rich-presence.
- :save! / :w! to overwrite a playlist. Plain :save now refuses to
  clobber an existing name.

Change
- Download audio with yt-dlp into the cache dir and play the local file
  (mpv --ytdl=no) instead of streaming. Keep the last 40 files and
  prefetch the next queued track.
- show the the actual yt-dlp/mpv error on playback failure instead of the
  generic "update yt-dlp" hint; allow wider status messages (70 - 120 char).
- Split artists from YouTube's linked artist runs and yt-dlp's 'artists'
  list (new Track::artists) rather than guessing from the display string.
  Fallback splitter keeps names like "Tyler, The Creator" whole and
  matches separators case-insensitively. (shit)
- Restyle selection (accent bold text + bar) and tab bar (underlined
  active tab instead of pills)
- Share one ureq agent across yt Music requests

Fix
- Multiple drift instances: only the first (flock on session.lock) owns
  the session, and playlist saves merge with what other instances wrote.
- Collection picker no longer steals keys while typing.
- "Play next" now shifts `played` indices so shuffle history stays right.
- Leaving the queue for a playlist/result list marks the queue as done.
- Replace unwrap() in lyrics sorting with total_cmp (NaN-safe).

Cargo.toml

@@ -16,6 +16,7 @@ toml = "0.8"
 ureq = { version = "2", features = ["json"] }
 dirs = "5"
 libc = "0.2"
+discord-rich-presence = "1.1.0"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 souvlaki = { version = "0.8", default-features = false, features = ["use_zbus"] }