feat(plex): Collections Phase 2 — admin collection editing (write-back to Plex)

Admins can curate Plex collections from a movie's info page ("Collections"
button → PlexCollectionEditor dialog): create a collection (seeded with the
movie), add/remove the movie to/from editable collections, delete a collection,
and "take over" an existing plain Plex collection (mark it editable). All writes
go to Plex (POST/PUT/DELETE via new PlexClient methods) and are reflected on
every client; a targeted single-collection re-sync (sync.resync_collection /
delete_collection_local) updates the local mirror without the full ~4-min sync.

Gating (per the design decisions): editing is ADMIN-ONLY (collections are shared
library-wide); only plain manual collections are editable — smart + external
auto-lists (IMDb/TMDb/…) are always read-only (can_edit = editable && !smart &&
source=="collection"). New admin endpoints under /api/plex/collections
(create/items add+remove/rename/delete/editable). Verified end-to-end incl. the
live Plex write API (create/add/rename/remove/delete all 200, self-cleaned) and
the editor UI (create + delete with confirm) on localdev.

frontend/src/components/PlexBrowse.tsx

@@ -146,6 +146,7 @@ export default function PlexBrowse({ q, onClearSearch, library, show, sort, filt
     return (
       <PlexInfoView
         id={infoId}
+        library={library}
         onBack={sub.back}
         onPlay={() => sub.open({ kind: "player", id: infoId })}
         onPlayItem={(id) => sub.open({ kind: "player", id })}
@@ -366,12 +367,14 @@ function PlexPosterCard({
 
 function PlexInfoView({
   id,
+  library,
   onBack,
   onPlay,
   onPlayItem,
   onFilter,
 }: {
   id: string;
+  library: string;
   onBack: () => void;
   onPlay: () => void;
   onPlayItem: (id: string) => void;
@@ -397,9 +400,11 @@ function PlexInfoView({
           <PlexInfo
             detail={q.data}
             variant="page"
+            library={library}
             onPlay={onPlay}
             onPlayItem={onPlayItem}
             onFilter={onFilter}
+            onStateChange={() => q.refetch()}
           />
         </Suspense>
       )}