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/lib/api.ts

@@ -344,6 +344,8 @@ export const api = {
     }),
   watchLaterRemove: (videoId: string) =>
     req(`/api/playlists/watch-later/${videoId}`, { method: "DELETE" }),
+  syncYoutubePlaylists: (): Promise<{ synced: number; reason?: string }> =>
+    req("/api/playlists/sync-youtube", { method: "POST" }),
 
   // --- quota usage ---
   myUsage: (): Promise<MyUsage> => req("/api/quota/my-usage"),