The detail pane rendered from react-query's cached data, so deleting the selected
playlist emptied the rail but left its stale contents on the right. Gate the detail
view on the current selection so it falls back to the empty state when nothing (or a
just-deleted playlist) is selected.
Add the Playlists page (left rail of playlists + detail with drag&drop reorder,
inline rename, one-step delete, remove item, Play all / row-click playback via the
existing PlayerModal) and an AddToPlaylist popover (portaled, multi-toggle +
inline new-playlist) wired into the VideoCard hover overlay and the PlayerModal.
New api client methods + Playlist types, a 'playlists' page route + account-menu
entry, and trilingual strings. Local only — YouTube sync comes in later phases.
