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/Feed.tsx

@@ -550,7 +550,7 @@ export default function Feed({
               onResetState={onResetState}
               onOpen={openVideo}
               hasNextPage={false}
-              isFetchingNextPage={false}
+              isFetching={false}
               fetchNextPage={() => {}}
             />
             {ytQuery.hasNextPage && (
@@ -826,8 +826,8 @@ export default function Feed({
           it stays put while the cards scroll (bug 4). On the feed page that slot is the shell's top
           band; on a channel page ChannelPage overrides the slot with its own sticky sub-header, so
           the toolbar rides with the tabs. Channel pages get no chips: their Feed starts from its OWN
-          baseline (channel-scoped, whole-catalog, show-all), so each would read as a "filter you
-          applied" when it's just the page being itself. */}
+          baseline (channel-scoped, whole-catalog, the watch state the channel page remembers), so
+          each would read as a "filter you applied" when it's just the page being itself. */}
       <PageToolbar>
         <div className={channelScoped ? "px-4 sm:px-6" : "px-4 pt-2 pb-1"}>
           {toolbar}
@@ -892,7 +892,7 @@ export default function Feed({
           onResetState={onResetState}
           onOpen={openVideo}
           hasNextPage={!!hasNextPage}
-          isFetchingNextPage={isFetchingNextPage}
+          isFetching={query.isFetching}
           fetchNextPage={fetchNextPage}
         />
       )}