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/VirtualFeed.tsx

@@ -29,7 +29,7 @@ export interface VirtualFeedProps {
   onResetState: (id: string) => void;
   onOpen: (v: Video, startAt?: number | null) => void;
   hasNextPage: boolean;
-  isFetchingNextPage: boolean;
+  isFetching: boolean;
   fetchNextPage: (options: typeof NEXT_PAGE) => unknown;
 }
 
@@ -44,7 +44,7 @@ export default function VirtualFeed({
   onResetState,
   onOpen,
   hasNextPage,
-  isFetchingNextPage,
+  isFetching,
   fetchNextPage,
 }: VirtualFeedProps) {
   const { minCol, colMaxWidth, maxCols, thumb } = FEED_VIEW_SPEC[view];
@@ -64,7 +64,7 @@ export default function VirtualFeed({
         maxCols={maxCols}
         rowEst={ROW_EST[view]}
         hasNextPage={hasNextPage}
-        isFetchingNextPage={isFetchingNextPage}
+        isFetching={isFetching}
         fetchNextPage={fetchNextPage}
         keyboardNav
         renderItem={(v, itemWidth, index) => (