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.contract.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from "vitest";
+
+import { eslint } from "./lintProbe.testkit";
+
+// eslint.config.js: a bare `fetchNextPage()` cancels the page already loading and sends it again
+// (E31 S4e). Pins that the rule keeping every call on NEXT_PAGE actually fires.
+const MESSAGE = "fetchNextPage() cancels";
+
+async function hits(source: string): Promise<number> {
+  const [result] = await eslint.lintText(source, { filePath: "src/components/__probe.tsx" });
+  return (result?.messages ?? []).filter((m) => m.message.startsWith(MESSAGE)).length;
+}
+
+describe("a next-page fetch", () => {
+  it("is refused bare, on the query or destructured", async () => {
+    expect(
+      await hits("export const a = (q: { fetchNextPage(): void }) => q.fetchNextPage();\n"),
+    ).toBe(1);
+    expect(await hits("export const b = (fetchNextPage: () => void) => fetchNextPage();\n")).toBe(
+      1,
+    );
+  });
+
+  it("passes with NEXT_PAGE", async () => {
+    expect(
+      await hits(
+        'import { NEXT_PAGE } from "../lib/nextPage";\nexport const c = (q: { fetchNextPage(o: unknown): void }) => q.fetchNextPage(NEXT_PAGE);\n',
+      ),
+    ).toBe(0);
+  });
+});