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.
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