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

static/locales/en.json

@@ -24,6 +24,7 @@
         "create_playlist": "Create Playlist",
         "playlists": "Playlists",
         "no_playlists": "No playlists yet",
+        "empty_hint": "Create your first playlist to start organizing your music",
         "select_playlist": "Select a playlist",
         "select_hint": "Choose a playlist from the sidebar or create a new one",
         "add_tracks": "Add Tracks",
@@ -62,7 +63,23 @@
         "artist": "Artist",
         "album": "Album",
         "tracks": "tracks",
-        "error": "Error"
+        "share_with_user": "User ID or email",
+        "playback_error": "Playback failed",
+        "error": "Error",
+        "remove": "Remove",
+        "track_removed": "Track removed",
+        "manage_shares": "Manage Shares",
+        "no_shares": "No shares yet",
+        "remove_share": "Remove share",
+        "can_write": "Can edit",
+        "read_only": "Read only",
+        "public": "Public",
+        "private": "Private",
+        "toggle_public": "Visibility",
+        "make_public": "Make public",
+        "make_private": "Make private",
+        "set_cover": "Set cover",
+        "cover_updated": "Cover updated"
     },
     "actions": {
         "search": "Search files...",