perf(ui): a next-page fetch joins the one in flight instead of sending the cursor again

react-query's fetchNextPage cancels a next page still loading and sends it again by default.
Our requests ignore the abort signal, so the server ran both: the feed's read-ahead effect
re-runs on scroll frames before isFetchingNextPage reaches it, and on prod every feed page
went out 2-3 times (a 0.8 s title sort queued to 2.3 s). Every call now passes NEXT_PAGE
({cancelRefetch: false}), lint-enforced, with a contract test.

frontend/src/lib/nextPage.dom.test.ts

@@ -0,0 +1,35 @@
+// @vitest-environment jsdom
+//
+// A next-page call while that page is still loading joins it instead of sending it again (E31 S4e):
+// on prod every feed page went out 2–3 times with the same cursor.
+import { InfiniteQueryObserver } from "@tanstack/react-query";
+import { describe, expect, it } from "vitest";
+
+import { NEXT_PAGE } from "./nextPage";
+import { createQueryClient } from "./queryClient";
+
+describe("NEXT_PAGE", () => {
+  it("sends each page once when the read-ahead calls twice in a row", async () => {
+    const sent: (string | null)[] = [];
+    const observer = new InfiniteQueryObserver(createQueryClient(), {
+      queryKey: ["probe"],
+      initialPageParam: null as string | null,
+      queryFn: async ({ pageParam }) => {
+        sent.push(pageParam);
+        await new Promise((r) => setTimeout(r, 5));
+        return { next: `c${sent.length}` };
+      },
+      getNextPageParam: (last: { next: string }) => last.next,
+    });
+    const unsubscribe = observer.subscribe(() => {});
+    await observer.refetch();
+
+    // Two scroll frames before `isFetchingNextPage` reaches the component.
+    void observer.fetchNextPage(NEXT_PAGE);
+    await observer.fetchNextPage(NEXT_PAGE);
+
+    expect(sent).toEqual([null, "c1"]);
+    expect(observer.getCurrentResult().data?.pages).toHaveLength(2);
+    unsubscribe();
+  });
+});