feat(music): Switch EMMS from MPD to MPV backend

Major improvements to music player configuration:

Backend Changes:
- Switch from emms-player-mpd to emms-player-mpv
- Remove MPD daemon dependency (no service management required)
- Configure MPV with --quiet, --no-video, --audio-display flags
- Support both local files and streaming URLs (http/https/mms)

Keybinding Enhancements:
- Add C-; m n → next track
- Add C-; m p → previous track
- Add C-; m g → go to playlist
- In playlist: n/P for next/prev, f/b for seek forward/backward
- Update which-key descriptions

Code Quality:
- Use music-dir constant from user-constants.el (not hardcoded path)
- Add (require 'user-constants) for proper dependency
- Update commentary to reflect MPV backend

Test Fixes:
- Fix 8 failing append-track tests
- Update test mock data to use cj/music-root for portability
- All 104 music-config tests now passing

Benefits:
- No daemon to start/stop/manage
- Simpler architecture (one process vs MPD+EMMS)
- Streaming radio URLs work out of the box
- Better path consistency across codebase

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