fix(library): a listing poll can no longer put back an older resume point

A poll already out when the player closed answered with the point the server held before the
closing beacon, over the row's patch; Continue then opened there and wrote it back. The close now
cancels the listings' fetches before patching and reads them again once the beacon has landed.

frontend/src/components/LibraryPlayer.dom.test.tsx

@@ -198,6 +198,36 @@ describe("LibraryPlayer", () => {
     expect(rows.map((j) => j.position_seconds)).toEqual([125, undefined]);
     // Closing the player sends the point the way that survives the page going away.
     unmount();
-    expect(beacon).toHaveBeenCalledWith(7, 125.4, 600);
+    expect(beacon).toHaveBeenCalledWith(7, 125.4, 600, expect.any(Function));
+  });
+
+  it("keeps the closing point over a list fetch that read the server before the point landed", async () => {
+    // The listing polls. One already out when the player closed answered with the point the server
+    // held BEFORE the closing beacon, over the row's patch — and Continue then opened there, and its
+    // first checkpoint wrote that older point back over the right one.
+    let settled: (() => void) | undefined;
+    vi.spyOn(api, "saveDownloadProgressBeacon").mockImplementation((_id, _at, _dur, onSettled) => {
+      settled = onSettled;
+    });
+    const client = new QueryClient();
+    const job = withAsset({ width: 1280, height: 720 });
+    client.setQueryData(qk.downloads(), [{ ...job, position_seconds: 10 }]);
+    let answer: (jobs: DownloadJob[]) => void = () => {};
+    const poll = client.fetchQuery({
+      queryKey: qk.downloads(),
+      queryFn: () => new Promise<DownloadJob[]>((r) => (answer = r)),
+    });
+    const { unmount } = render(<LibraryPlayer job={job} onClose={() => {}} />, client);
+    const video = loaded();
+    video.dispatchEvent(new Event("playing"));
+    video.currentTime = 300;
+    unmount();
+    answer([{ ...job, position_seconds: 10 }]);
+    await poll;
+    expect(client.getQueryData<DownloadJob[]>(qk.downloads())![0]!.position_seconds).toBe(300);
+    // Once the point has landed, the listing asks the server again.
+    const invalidate = vi.spyOn(client, "invalidateQueries");
+    settled?.();
+    expect(invalidate).toHaveBeenCalledWith({ queryKey: qk.downloads(), exact: true });
   });
 });