feat(music): native music core - shared helpers, album folder template, display cleanup (#125)

* Add native Music core workflow

* feat(music): add shared music core helpers, album folder template, and UX cleanup

Consolidate duplicated music-service logic into core and extend the native music workflow.

- add core/music/extract.py: shared stateless helpers (first_text, first_number, year/duration/name formatting, classify_release_kind, dedupe_track_options, build_music_from_songs) so services stop carrying their own copies
- add core/music/display.py: shared rich rendering (render_track_panel, render_album_header, render_artwork_preview) with TrackRow / MusicHeaderInfo data holders
- export the new helpers from core/music/__init__.py
- add dedicated `albums` folder template kind (output_template.folder.albums) resolving albums -> songs -> "{artist} - {album} ({year})"; whitelist music template variables (album_artist, track_total, disc_total, release_type, genre, explicit, isrc, upc, label)
- fix song filename crash: config.get_output_template(...) did not exist; use config.output_template.get("songs") with a sane default
- strip emojis from music output (renderer, dl.py music branch) to match unshackle UX; remove dead MusicSongPlan import and music_icon logic
- document the albums folder key in unshackle-example.yaml
- add tests for extract, display, and the folder template

---------

Co-authored-by: MrMovies-Dev <MrMovies-Dev@users.noreply.github.com>

uv.lock

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