fix: speaker recovers from the wrong song, stalls and duplicate tabs

[madeofpendletonwool]

What happened

On 2026-10-15 in The Work Boys, with a friend's iPhone (Chrome, iOS 26) playing to a Bluetooth speaker:

1. Feel It Still played over Helena Beat, and kept going until a pause then play from the big screen. Navidrome's logs show a second player starting up about a minute into Helena Beat (12:43:43 and again at 12:44:06 CDT). It loaded Helena Beat and preloaded Feel It Still, on top of the iPhone's normal requests.
2. Roxanne (Spotify, transcoded for iOS) went silent after a skip onto its preloaded stream, but still showed as playing. It's still marked as playing in the DB. Prod logged no automatic playback notices that day, so the server never timed it out.

The exact device actions couldn't be reconstructed, and the server didn't log playback events. So this PR fixes the client-side gaps that lead to those symptoms, and adds logging.

Changes

• Wrong song: offSong() notices when a device is on another song than the room at the same revision, and apply loads the room's song. Before, only a new revision corrected it, which is why pause then play fixed it. The gapless handoff (the next song in the last 10s of the room's) is left alone.
• Stall watchdog: if the position hasn't moved for 10s while the room plays, the song reloads where it was. After two in a row, the speaker reports error. Reloading back to the stall point doesn't count as progress.
• One player per browser: tabs share the device ID, so the newest to play tells the others to go quiet over a BroadcastChannel. If the other tab played as a different device, it releases instead.
• begin() clears curItem/preItem, since it puts silence in both elements. Starting while already active could play silence and skip songs.
• Server logs (Info): speaker claimed and released, commands, next song with its reason, and reports. Progress reports are logged only when stale.
• Docs: docs/player-mode.md.

Testing

• go test ./internal/playback ./internal/httpapi pass
• Web: 309 tests pass (new offSong cases), oxlint and tsc clean
• Not yet checked in a browser or on a device. Worth a run with an iPhone speaker locked through several songs, including a skip onto a Spotify song.

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