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/eslint.config.js

@@ -547,6 +547,15 @@ export default tseslint.config(
       // lib/*.contract.test.ts).
       "no-restricted-syntax": [
         "error",
+        // A next-page fetch called bare CANCELS the one in flight and sends the same cursor again;
+        // our requests ignore the abort, so the server ran both (E31 S4e: every feed page 2–3×).
+        // lib/nextPage.ts NEXT_PAGE makes the second call join the first.
+        {
+          selector:
+            "CallExpression[arguments.length=0]:matches([callee.name='fetchNextPage'], [callee.property.name='fetchNextPage'])",
+          message:
+            "fetchNextPage() cancels and re-sends a page already loading: call fetchNextPage(NEXT_PAGE) (lib/nextPage.ts).",
+        },
         // `enabled:` sees only the native attribute; a FocusSafeButton that holds the focus is
         // disabled by aria-disabled, so a bare `enabled:hover:` lit it up on hover as if it worked.
         {