Music player: rail controls, now-playing equalizer, folder-linked playlists, auto-advance

[DevMando]

Summary

Adds a music player to the Desktop — player controls on the left rail, an animated now-playing indicator, and user-defined playlists — and rolls the engine pin to pick up the fix that makes music play through a playlist instead of going silent after one song. The code was reviewed by four independent cleanup passes (reuse, simplification, efficiency, architecture) and refactored accordingly before this description was written.

What's new (plain language)

1. A music player on the left rail. Clicking the music icon opens a small panel: play/pause, next, stop, a volume slider, and a playlist picker. No page navigation — it works like a system-tray player.

2. You can see that music is playing, and what. While a song plays, the rail icon becomes a small animated gold equalizer (the Desktop's answer to the CLI's title-bar visualizer). Hovering it names the current song. The indicator stays truthful no matter how playback changes — panel buttons, /music chat commands, songs advancing on their own, or an audio-device failure — and any playback error is shown in the panel the moment it happens.

3. Bring-your-own playlists. "Add playlist" points the player at any folder of MP3s on your disk. Nothing is copied or moved — the app creates a lightweight folder link under ~/.mandocode/music, which means the CLI sees the same playlists too. The new playlist is selected immediately; re-adding a folder that's already a playlist just selects the existing one (no duplicates). "Remove" deletes only the link, never your files, and only ever appears for playlists added this way. Your selected playlist and volume both survive an app restart.

4. Music now plays through the playlist (engine fix). Previously a song played once and then went silent forever while the player still claimed to be playing — in both the CLI and Desktop. The engine now detects the end of a song and advances to a random next track (a one-song playlist repeats). Fixed upstream in MandoCode PR #67; this PR rolls the pin to pick it up.

Scope / risk

• New UI is additive: one rail button + flyout, one code-behind file, and a small playlist service. No changes to chat, agents, or approvals.
• Playlist operations only ever create or delete folder links under ~/.mandocode/music; deleting a playlist structurally cannot touch the music files it points at.
• The engine pin roll contains exactly the music fix (verified: two commits, one file + changelog).
• Known limitation: playlist folders must be on a local disk (no network shares) — a constraint of admin-free folder links.

Verification

• Built clean (0 warnings) on every commit; engine test suite 486/486 on the pinned commit.
• Manually verified end-to-end: a finished track flows into the next; the equalizer animates only while audio actually plays; /music stop from chat settles the icon within ~2 seconds; adding a playlist selects it and switches playback; re-adding the same folder reuses the existing playlist; removing a playlist leaves the source folder untouched; the playlist pick survives closing and reopening the panel.
• Post-review hardening: playlist file operations run off the UI thread; a playlist pointing at an unplugged drive can no longer stall the window; the state poll stops when the window closes.