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.

backend/app/routes/playlists.py

@@ -31,7 +31,13 @@ def _mark_dirty(pl: Playlist) -> None:
         pl.dirty = True
 
 
-def _summary(db: Session, pl: Playlist, count: int, has_video: bool | None = None) -> dict:
+def _summary(
+    db: Session,
+    pl: Playlist,
+    count: int,
+    total_duration: int = 0,
+    has_video: bool | None = None,
+) -> dict:
     cover = db.scalar(
         select(Video.thumbnail_url)
         .join(PlaylistItem, PlaylistItem.video_id == Video.id)
@@ -47,6 +53,7 @@ def _summary(db: Session, pl: Playlist, count: int, has_video: bool | None = Non
         "yt_playlist_id": pl.yt_playlist_id,
         "dirty": pl.dirty,
         "item_count": count,
+        "total_duration_seconds": total_duration,
         "cover_thumbnail": cover,
     }
     if has_video is not None:
@@ -54,6 +61,18 @@ def _summary(db: Session, pl: Playlist, count: int, has_video: bool | None = Non
     return out
 
 
+def _total_duration(db: Session, playlist_id: int) -> int:
+    return (
+        db.scalar(
+            select(func.coalesce(func.sum(Video.duration_seconds), 0))
+            .select_from(PlaylistItem)
+            .join(Video, Video.id == PlaylistItem.video_id)
+            .where(PlaylistItem.playlist_id == playlist_id)
+        )
+        or 0
+    )
+
+
 @router.get("")
 def list_playlists(
     contains: str | None = None,
@@ -81,6 +100,17 @@ def list_playlists(
             .group_by(PlaylistItem.playlist_id)
         ).all()
     )
+    durations = dict(
+        db.execute(
+            select(
+                PlaylistItem.playlist_id,
+                func.coalesce(func.sum(Video.duration_seconds), 0),
+            )
+            .join(Video, Video.id == PlaylistItem.video_id)
+            .where(PlaylistItem.playlist_id.in_(ids))
+            .group_by(PlaylistItem.playlist_id)
+        ).all()
+    )
     member: set[int] = set()
     if contains:
         member = set(
@@ -94,7 +124,13 @@ def list_playlists(
             .all()
         )
     return [
-        _summary(db, p, counts.get(p.id, 0), (p.id in member) if contains else None)
+        _summary(
+            db,
+            p,
+            counts.get(p.id, 0),
+            durations.get(p.id, 0),
+            (p.id in member) if contains else None,
+        )
         for p in pls
     ]
 
@@ -339,7 +375,7 @@ def rename_playlist(
     count = db.scalar(
         select(func.count()).where(PlaylistItem.playlist_id == pl.id)
     )
-    return _summary(db, pl, count or 0)
+    return _summary(db, pl, count or 0, _total_duration(db, pl.id))
 
 
 @router.delete("/{playlist_id}")