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.dom.test.tsx

@@ -38,7 +38,13 @@ vi.mock("../lib/notifications", async (importOriginal) => ({
 }));
 
 import PlexCollectionView from "./PlexCollectionView";
-import { api, type PlexCard, type PlexCollectionDetail, type PlexShowDetail } from "../lib/api";
+import {
+  api,
+  HttpError,
+  type PlexCard,
+  type PlexCollectionDetail,
+  type PlexShowDetail,
+} from "../lib/api";
 
 afterEach(() => {
   cleanup();
@@ -135,12 +141,24 @@ describe("PlexCollectionView — after the sort joined the query key", () => {
   it("flips a poster's watched state on the page it was clicked on", async () => {
     // The optimistic write kept the key's OLD spelling — an entry nothing reads — and the resync
     // only touches the library root, so the click did nothing visible until a reload.
-    vi.spyOn(api, "plexSetState").mockResolvedValue(undefined as never);
+    // The request never answers, so the resync never runs: only the optimistic write can flip it.
+    vi.spyOn(api, "plexSetState").mockReturnValue(new Promise(() => {}));
     mount([card("movie1", "movie")]);
     fireEvent.click(await screen.findByRole("button", { name: "plex.markWatched" }));
     expect(await screen.findByRole("button", { name: "plex.markUnwatched" })).toBeTruthy();
   });
 
+  it("puts a toggle that failed back the way it was", async () => {
+    // With the flip landing on the entry the page reads, the resync is what corrects it — and it only
+    // touched the library root, so a failed toggle stayed "watched" beside its own error toast.
+    vi.spyOn(api, "plexSetState").mockRejectedValue(new HttpError(422, "nope"));
+    mount([card("movie1", "movie")]);
+    fireEvent.click(await screen.findByRole("button", { name: "plex.markWatched" }));
+    await waitFor(() => expect(api.plexCollection).toHaveBeenCalledTimes(2));
+    expect(await screen.findByRole("button", { name: "plex.markWatched" })).toBeTruthy();
+    expect(screen.queryByRole("button", { name: "plex.markUnwatched" })).toBeNull();
+  });
+
   it("keeps the page, and the open Sort panel, on screen while the new order loads", async () => {
     // Every sort change was a key with no data yet: the whole page — the control and its open panel
     // included — was swapped for a spinner, so key and direction could not be set in one visit.

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() {