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/ui/DataTable.tsx

@@ -274,10 +274,12 @@ export default function DataTable<T>({
     if (at >= 0 && !allRows) setPage(Math.floor(at / size));
   }
   const paged = allRows ? sorted : sorted.slice(safePage * size, safePage * size + size);
-  // A long page ("All", 100) mounts its first rows at once and the rest in the background.
+  // A long page ("All", 100) mounts its first rows at once and the rest in the background — but
+  // always through the revealed row, which the caller focuses as soon as this frame commits.
   const shown = useProgressiveRows(
     paged,
     `${safePage}:${size}:${JSON.stringify(sort)}:${JSON.stringify(filters)}:${narrowedBy}`,
+    reveal ? paged.findIndex((row) => rowKey(row) === reveal.key) : -1,
   );
 
   function toggleSort(key: string) {