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/PlexBrowse.tsx

@@ -9,6 +9,7 @@ import {
 } from "@tanstack/react-query";
 import { User, X } from "lucide-react";
 import { qk } from "../lib/queryKeys";
+import { NEXT_PAGE } from "../lib/nextPage";
 import { healPlexField } from "../lib/plexSearch";
 import { healPlexSort, plexSortOptions, PLEX_SORT_FALLBACK } from "../lib/plexSort";
 import type { SortState } from "../lib/columnSort";
@@ -321,7 +322,8 @@ export default function PlexBrowse() {
     if (!sentinelEl) return;
     const io = new IntersectionObserver(
       (entries) => {
-        if (entries[0]?.isIntersecting && hasNextPage && !isFetchingNextPage) fetchNextPage();
+        if (entries[0]?.isIntersecting && hasNextPage && !isFetchingNextPage)
+          void fetchNextPage(NEXT_PAGE);
       },
       { rootMargin: "600px" },
     );