The old per-video status='saved' becomes membership in a built-in, undeletable
'watch_later' playlist. Migration 0012 moves existing saved videos into each user's
Watch later list and demotes the states to 'new'. The feed/playlist serializers now
expose a 'saved' boolean (EXISTS in watch_later); the card bookmark toggles watch_later
via new /api/playlists/watch-later add/remove endpoints (create-on-demand) instead of
setting a status. Removed the 'saved' show-filter and status. Watch later shows a
localized name and hides rename/delete in the Playlists page and add-to-playlist popover.
Adding a video to a playlist via the AddToPlaylist popover invalidated the membership
and the playlist list, but not the playlist *detail* query, so the Playlists page
served stale cached items (the new video only appeared after F5). Invalidate the
["playlist"] detail queries too.
The popover was always positioned below its trigger, so on cards near the bottom
of the viewport it ran off-screen. Measure the panel's real height and open it
upward (clamped to the viewport) when there isn't room below; also clamp the
horizontal position.
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.
