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).

src/session.rs

@@ -22,6 +22,37 @@ impl Session {
         Ok(dir.join("drift").join("session.json"))
     }
 
+    /* The first drift to start owns the saved session. Later ones neither
+    restore nor overwrite it. The lock goes away when the process exits. */
+    #[cfg(unix)]
+    pub fn claim() -> bool {
+        use std::os::unix::io::AsRawFd;
+        let Ok(path) = Self::path().map(|p| p.with_file_name("session.lock")) else {
+            return true;
+        };
+        if let Some(dir) = path.parent() {
+            let _ = fs::create_dir_all(dir);
+        }
+        let Ok(file) = fs::OpenOptions::new()
+            .create(true)
+            .truncate(false)
+            .write(true)
+            .open(&path)
+        else {
+            return true;
+        };
+        let ok = unsafe { libc::flock(file.as_raw_fd(), libc::LOCK_EX | libc::LOCK_NB) } == 0;
+        if ok {
+            std::mem::forget(file);
+        }
+        ok
+    }
+
+    #[cfg(not(unix))]
+    pub fn claim() -> bool {
+        true
+    }
+
     pub fn exists() -> bool {
         Self::path().is_ok_and(|p| p.exists())
     }
@@ -75,6 +106,7 @@ mod tests {
             id: id.into(),
             title: "T".into(),
             artist: "A".into(),
+            artists: Vec::new(),
             album: None,
             duration: Some(200),
         }