feat(music): music browsing UI — square cards, library groupings, artist/album screens

Un-gates music libraries; adds CardShape.square plumbing (circular
artists), artists/albums/tracks browse groupings with per-grouping
sorts, audio playlists tab, artist + album detail screens with dpad
track rows, music context-menu actions, and a stub MusicPlaybackService
seam for the upcoming engine.

lib/i18n/en.i18n.json

@@ -784,6 +784,9 @@
       "shows": "TV Shows",
       "seasons": "Seasons",
       "episodes": "Episodes",
+      "artists": "Artists",
+      "albums": "Albums",
+      "tracks": "Tracks",
       "folders": "Folders"
     },
     "filterCategories": {
@@ -970,6 +973,18 @@
     "errorReordering": "Failed to reorder playlist item",
     "errorRemoving": "Failed to remove from playlist"
   },
+  "music": {
+    "goToAlbum": "Go to album",
+    "goToArtist": "Go to artist",
+    "instantMix": "Instant Mix",
+    "playNext": "Play next",
+    "addToQueue": "Add to queue",
+    "discNumber": "Disc ${n}",
+    "trackCount": {
+      "one": "${n} track",
+      "other": "${n} tracks"
+    }
+  },
   "watchTogether": {
     "title": "Watch Together",
     "description": "Watch content in sync with friends and family",