feat: playlist support (POC)

[velocitysystems]

Summary

Adds playlist support to the Rodio-backed audio player.
Auto-extracts ID3 metadata from loaded items to surface title, artist, and embedded artwork.
Replaces the single-channel state-update event with four typed event channels.

API changes

Breaking

• load(src, metadata) → load(playlist, startIndex?) — the single-source signature is gone. Callers now pass PlaylistItem[] (each item is { src, metadata? }). For the previous single-track use case, pass a one-item array.
• setLoop(loop: boolean) → setLoopMode(mode: LoopMode) — boolean replaced with a LoopMode enum (Off / One / All). The Tauri permission rename matches: allow-set-loop → allow-set-loop-mode.
• player.listen(...) removed — replaced by four typed event channels (see below).
• PlayerState.title / PlayerState.artist removed — per-track metadata now lives on PlayerState.playlist[currentIndex].metadata.
• PlayerState.playlist and PlayerState.currentIndex added — currentIndex is null when Idle.

Added

• Transport actions: next(), prev(), jumpTo(index). Allowed in Ready / Playing / Paused / Ended / Error (Error allows them so consumers can skip past a broken track without reloading the whole playlist). prev() restarts the current item if currentTime > 3s, otherwise moves to the previous item (with wrap-around in LoopMode.All).
• LoopMode enum (Off / One / All) — auto-advance behaviour at end of track or end of playlist.
• PlaylistItem interface — { src: string, metadata?: AudioMetadata }.
• Event channels (replacing listen):
  • onStateChanged((change: StateChange) => void) — status / error transitions only.
  • onTrackChanged((change: TrackChange) => void) — fires after each item finishes loading; payload carries the enriched playlist item (auto-extracted ID3 title / artist / artwork merged with caller-supplied metadata, caller wins per field).
  • onSettingsChanged((change: SettingsChange) => void) — volume / muted / playbackRate / loopMode deltas.
  • onTimeUpdate((time: TimeUpdate) => void) — high-frequency position updates (~250 ms) plus seek hits.
• PlaybackStatus.Error is now in allowedActions for Load, Stop, Next, Prev, JumpTo (the recovery surface).
• Tauri commands: next, prev, jump_to, set_loop_mode (replaces set_loop).