feat(playlists): YouTube-mirrored playlists in the UI (read-only) + sync button

Show YouTube-sourced playlists with a YouTube icon; they're read-only for now (no
rename/delete/reorder/remove, excluded from the add-to-playlist popover) with a
'synced from YouTube' note — editing comes with the write-back phase. Add a 'Sync from
YouTube' button in the Playlists rail (api.syncYoutubePlaylists) with a result toast.
Trilingual strings.

frontend/src/components/AddToPlaylist.tsx

@@ -118,7 +118,9 @@ export default function AddToPlaylist({
     }
   }
 
-  const lists = membership.data ?? [];
+  // YouTube-mirrored playlists are read-only until the write-back phase, so they can't be
+  // added to here yet.
+  const lists = (membership.data ?? []).filter((pl) => pl.source !== "youtube");
 
   return (
     <>