feat(playlists): rail sorting, consolidated item sort, persist selection

1) Fix: the selected playlist is now persisted (localStorage) and scrolled into view,
   so F5 keeps it instead of jumping to the first one.
2) Consolidate the in-detail sort: one key select (Title/Duration/Channel) + an
   asc/desc direction toggle (was separate A-Z / Z-A / shortest / longest options);
   add Channel as a sort key. Direction also orders the channel groups when grouping.
3) Left rail sorting: by name / item count / total length, asc/desc, plus an
   'unsynced first' toggle that floats playlists with unpushed edits to the top.
   Backend: list/summary now return total_duration_seconds (grouped sum). The rail
   also shows each playlist's total length. Sort prefs persist to localStorage.

frontend/src/lib/api.ts

@@ -77,6 +77,7 @@ export interface Playlist {
   yt_playlist_id: string | null;
   dirty: boolean; // linked local playlist has edits not yet pushed to YouTube
   item_count: number;
+  total_duration_seconds: number;
   cover_thumbnail: string | null;
   has_video?: boolean; // only set when listed with ?contains=<video_id>
 }