fix(ui): a read-ahead waits out a background refetch, and a revealed row is mounted before it is focused

NEXT_PAGE joins ANY fetch in flight: a read-ahead during an invalidation
or a poll joined that refetch and appended no page, and nothing asked
again until the reader moved the range (the channel page during a
backfill). The read-ahead now waits while the list fetches anything
(mayReadAhead, re-run on isFetching); a Load older click cancels a poll
instead (nextPageOnClick).

A page turn to a revealed row restarted the progressive mount at 30
rows; Channels focuses that row in the next commit, and at index 40 of
a 50-row page it was not there, so focus fell to <body>. The mount now
reaches through the revealed row.

frontend/src/components/AdminLogs.tsx

@@ -13,7 +13,7 @@ import { api, type LogFacet, type LogQuery, type ServerLogRow } from "../lib/api
 import { copyText } from "../lib/clipboard";
 import { formatDateTimeSec } from "../lib/format";
 import { nextNavId, notifyNav, rememberNavPosition } from "../lib/history";
-import { NEXT_PAGE } from "../lib/nextPage";
+import { nextPageOnClick } from "../lib/nextPage";
 import { LEVEL_GROUPS, type LevelGroup } from "../lib/log";
 import {
   DEFAULT_LOGS_STATE,
@@ -157,7 +157,8 @@ export default function AdminLogs() {
           ? "countsFailed"
           : null;
   const retryFailed = () => {
-    if (logs.isFetchNextPageError) void logs.fetchNextPage(NEXT_PAGE);
+    if (logs.isFetchNextPageError)
+      void logs.fetchNextPage(nextPageOnClick(logs.isFetchingNextPage));
     else if (logs.isError) void logs.refetch();
     if (facets.isError) void facets.refetch();
   };
@@ -191,7 +192,7 @@ export default function AdminLogs() {
 
   const loadOlder = () => {
     setLive(false);
-    void logs.fetchNextPage(NEXT_PAGE);
+    void logs.fetchNextPage(nextPageOnClick(logs.isFetchingNextPage));
   };
 
   const copy = async () => {