fix(plex): a collection's failed watched toggle is put back the way it was

C1 moved the optimistic flip onto the key the page reads — and so made visible that nothing corrects
it: the resync only invalidated `plex-library`, so a failed toggle stayed "watched" beside its own
error toast, and the collection's other cached sort orders stayed stale after a good one. The
resync now takes this page's `detail` prefix too, as the Discover and person pages do for theirs.
The C1 test holds the request open, so it still pins the optimistic write on its own.

frontend/src/components/PlexCollectionView.tsx

@@ -165,7 +165,14 @@ export default function PlexCollectionView({
             : old,
         );
       },
-      resync: () => qc.invalidateQueries({ queryKey: qk.plexLibrary() }),
+      // This page's OWN key too, not just the library's: the flip above lands on the entry the page
+      // reads, and resync is what corrects it to the truth when the toggle fails — invalidating only
+      // `plex-library` left a failed toggle flipped on screen. The prefix (no sort in it) also
+      // refreshes the other sort orders of this collection cached behind this one.
+      resync: () => {
+        qc.invalidateQueries({ queryKey: qk.plexLibrary() });
+        qc.invalidateQueries({ queryKey: qk.plexCollections("detail", collectionId) });
+      },
     });
 
   async function doRename() {