fix(ui): review round 2 — reveal floor, failed next page, count states, row Play

- DataTable: the reveal floor holds only in the view it was made for (Channels
  never clears its follow, so every later sort mounted rows up to it)
- Feed + Plex browse: a failed next page stops the read-ahead and shows a row
  that retries, instead of re-requesting it once a round-trip
- the YouTube Load more click uses nextPageOnClick; a contract pins NEXT_PAGE
  to the read-aheads and the click sites to nextPageOnClick
- ItemCount: a failed count says so; a stale one says updating to a screen reader
- the narrow-picture Play lift applies only where a hover row is there to clear
- PlayerModal: one description ladder for the popover and the panel, both tested
- plexImage contract reads every img src, any attribute order
- Tabs comment names who persists the sub-tab

frontend/src/components/PlexBrowse.tsx

@@ -9,7 +9,7 @@ import {
 } from "@tanstack/react-query";
 import { User, X } from "lucide-react";
 import { qk } from "../lib/queryKeys";
-import { mayReadAhead, NEXT_PAGE } from "../lib/nextPage";
+import { mayReadAhead, NEXT_PAGE, nextPageOnClick } from "../lib/nextPage";
 import { healPlexField } from "../lib/plexSearch";
 import { healPlexSort, plexSortOptions, PLEX_SORT_FALLBACK } from "../lib/plexSort";
 import type { SortState } from "../lib/columnSort";
@@ -317,7 +317,10 @@ export default function PlexBrowse() {
   // mount/unmount, so the observer always watches the live node; a plain ref would keep observing the
   // detached old node after Back and never fire fetchNextPage again (feed stuck at the loaded pages).
   const [sentinelEl, setSentinelEl] = useState<HTMLDivElement | null>(null);
-  const { hasNextPage, isFetching, isFetchingNextPage, fetchNextPage } = browseQ;
+  const { isFetching, isFetchingNextPage, isFetchNextPageError, fetchNextPage } = browseQ;
+  // A failed next page stops the read-ahead (the row at the bottom retries): the observer is rebuilt
+  // on every settle and would re-request it once a round-trip while the sentinel is in view.
+  const hasNextPage = browseQ.hasNextPage && !isFetchNextPageError;
   useEffect(() => {
     if (!sentinelEl) return;
     const io = new IntersectionObserver(
@@ -753,6 +756,15 @@ export default function PlexBrowse() {
       {isFetchingNextPage && (
         <p className="text-center text-xs text-muted pb-4">{t("plex.loading")}</p>
       )}
+      {isFetchNextPageError && !isFetchingNextPage && (
+        <StateMessage
+          tone="error"
+          compact
+          onRetry={() => void fetchNextPage(nextPageOnClick(isFetchingNextPage))}
+        >
+          {t("common.loadMoreFailed")}
+        </StateMessage>
+      )}
     </div>
   );
 }