Header shows collage, name, description, public/private chip, track
count, owner attribution (when not owner). Edit + delete buttons
visible to the owner only. Track list uses PlaylistTrackRow with
HTML5 drag-and-drop; drop fires PUT /tracks with the new ordered
positions and TanStack Query invalidates the playlist + index cache.
Toast surface is a placeholder browser alert in slice 1 — a real
toast is a polish task whenever it lands.