feat: scrobble playback for ListenBrainz/Last.fm integration (#1)

[klay2000]

Summary

• Closes Add ListenBrainz Integration #1 — "Add ListenBrainz Integration"
• Gonic already supports forwarding scrobbles to Last.fm/ListenBrainz (configured server-side in Gonic's admin settings), but only if the Subsonic client calls the standard /rest/scrobble endpoint. This client never did. This PR adds that call.
• Adds scrobble(id, { submission, time }) to src/lib/api/subsonic.js, following the existing request()/buildUrl() pattern. Best-effort — errors are swallowed and never interrupt playback.
• Adds a pure helper shouldSubmitScrobble(elapsedSeconds, durationSeconds) to src/lib/stores/playerLogic.js implementing the standard Last.fm/ListenBrainz threshold: a track counts as "played" once it's been listened to for 4 minutes OR 50% of its duration, whichever comes first.
• Wires both into PlayerBar.svelte:
  • On track change: fires a "now playing" scrobble (submission=false) and resets a per-track scrobbled flag.
  • In onTimeUpdate: once shouldSubmitScrobble returns true and the track hasn't been scrobbled yet, fires the final scrobble (submission=true) and sets the flag so it only fires once per track.

Test plan

• npm test passes (53/53, including 5 new tests for shouldSubmitScrobble covering the 4-minute cap, 50% threshold, and unknown-duration cases)
• npm run build passes
• Needs verification against a real Gonic instance (not possible in this sandbox — no server credentials): play a track and confirm
  • Gonic's scrobble logs show both a "now playing" and a final scrobble request
  • If Last.fm/ListenBrainz forwarding is configured in Gonic's admin settings, the track shows up as "now playing" on ListenBrainz, then gets scrobbled after the threshold

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