feat(music): complete music UI overhaul - bugs, UX, missing features & styling

- Fix dead buttons (fa-edit/fa-share → fa-pen/fa-share-alt matching icon registry)
- Fix volume icon, shuffle bias, queue removal, repeat-one CSS
- Replace native prompt/confirm/alert with Modal system
- Add click-to-select, dblclick-to-play, playback error notifications
- Add loading indicators, success toasts, close player button
- Implement remove track from playlist (DELETE endpoint)
- Implement drag & drop track reorder (PUT reorder endpoint)
- Implement manage shares dialog (GET/DELETE shares endpoints)
- Implement cover art upload & display (cover_file_id in UpdatePlaylistDto)
- Implement public/private toggle (is_public in UpdatePlaylistDto)
- Mount get_audio_metadata route in backend routes.rs
- Redesign empty state: full-width centered onboarding when 0 playlists
- Move create button into sidebar header as compact + button
- Add music.css to build.rs production CSS bundle
- Add 16+ i18n keys in en.json and es.json
- Add CSS for drag handles, track remove, cover overlay, shares dialog, public badge

build.rs

@@ -35,6 +35,7 @@ const INDEX_VIEW_CSS: &[&str] = &[
     "views/trash.css",
     "views/photos.css",
     "views/photosLightbox.css",
+    "views/music.css",
 ];
 
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