The searchable Collections picker was dropped from the sidebar when browsing
went unified (movies+shows across libraries) because collections are per-library
and there's no single selected library. Bring it back library-agnostic: GET
/api/plex/collections `library` is now optional — without it the endpoint unions
collections across all enabled libraries (with it, the old single-library path the
collection editor uses). The sidebar re-adds the searchable list (chip when one is
active). Its query key is ["plex-collections","union",<search>] so a search term
equal to a library plex_key can't collide with the editor's ["plex-collections",<key>],
while the shared prefix keeps editor invalidation refreshing the picker.
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