Watchlist membership was only reachable from Explore cards and the
detail screen's action row, which drops the bookmark first on narrow
screens with no fallback in the overflow menu. Add an entry to
MediaContextMenu for movies and shows whenever a connected catalog
source can hold the item, covering card long-press everywhere and the
detail screen's overflow.
External-id resolution is session-cached per item on
CatalogSourcesProvider and shared with the detail screen. A cold cache
labels the entry "Add to Watchlist" and always adds (idempotent), so a
press can never turn into a surprise removal; "Remove" is offered once
cached membership proves it. Several capable sources open the same
per-source chooser the detail screen uses.
close#1822
