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/i18n/locales/en/playlists.json

@@ -37,9 +37,15 @@
   "deleteYtFailed": "Couldn't delete on YouTube; removed here only.",
   "sortLabel": "Sort",
   "sortManual": "Custom order",
-  "sortTitleAsc": "Title A–Z",
-  "sortTitleDesc": "Title Z–A",
-  "sortDurAsc": "Shortest first",
-  "sortDurDesc": "Longest first",
-  "groupByChannel": "Group by channel"
+  "sortTitle": "Title",
+  "sortDuration": "Duration",
+  "sortChannel": "Channel",
+  "dirAsc": "Ascending",
+  "dirDesc": "Descending",
+  "groupByChannel": "Group by channel",
+  "railSortCustom": "Custom order",
+  "railSortName": "Name",
+  "railSortCount": "Item count",
+  "railSortDuration": "Total length",
+  "dirtyFirst": "Unsynced first"
 }