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.
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