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/components/ui/VirtualGrid.tsx

@@ -10,6 +10,7 @@ import {
 import { useVirtualizer } from "@tanstack/react-virtual";
 import { useListNav } from "../../lib/listNav";
 import { scrollParent } from "../../lib/listPaging";
+import { NEXT_PAGE } from "../../lib/nextPage";
 import { GRID_GAP, columnsFor, itemWidthFor } from "./gridColumns";
 import { shouldPrefetchNext } from "./readAhead";
 
@@ -66,7 +67,8 @@ export interface VirtualGridProps<T> {
   /** Infinite scroll — omit all three for a finite client-side list. */
   hasNextPage?: boolean;
   isFetchingNextPage?: boolean;
-  fetchNextPage?: () => void;
+  /** Called with `NEXT_PAGE`, so a read-ahead that fires again joins the page already loading. */
+  fetchNextPage?: (options: typeof NEXT_PAGE) => unknown;
 }
 
 export default function VirtualGrid<T>({
@@ -167,7 +169,7 @@ export default function VirtualGrid<T>({
     const last = virtualRows[virtualRows.length - 1];
     if (!last) return;
     if (shouldPrefetchNext(last.index, rows.length, virtualRows.length)) {
-      fetchNextPage();
+      fetchNextPage(NEXT_PAGE);
     }
   }, [virtualRows, rows.length, hasNextPage, isFetchingNextPage, fetchNextPage]);