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/es.json

@@ -24,6 +24,7 @@
         "create_playlist": "Crear Lista",
         "playlists": "Listas",
         "no_playlists": "Sin listas aún",
+        "empty_hint": "Crea tu primera lista para empezar a organizar tu música",
         "select_playlist": "Selecciona una lista",
         "select_hint": "Elige una lista de la barra lateral o crea una nueva",
         "add_tracks": "Añadir Pistas",
@@ -62,7 +63,23 @@
         "add_error": "No se pudieron añadir las pistas",
         "no_playlists_yet": "No hay listas aún. ¡Crea una primero!",
         "selected_files": "Seleccionados:",
-        "error": "Error"
+        "share_with_user": "ID de usuario o email",
+        "playback_error": "Error de reproducción",
+        "error": "Error",
+        "remove": "Eliminar",
+        "track_removed": "Pista eliminada",
+        "manage_shares": "Gestionar compartidos",
+        "no_shares": "Sin compartidos aún",
+        "remove_share": "Eliminar compartido",
+        "can_write": "Puede editar",
+        "read_only": "Solo lectura",
+        "public": "Pública",
+        "private": "Privada",
+        "toggle_public": "Visibilidad",
+        "make_public": "Hacer pública",
+        "make_private": "Hacer privada",
+        "set_cover": "Establecer portada",
+        "cover_updated": "Portada actualizada"
     },
     "share": {
         "dialogTitle": "Compartir Enlace",