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/ui/ItemCount.tsx

@@ -14,15 +14,26 @@ const OVERFLOW_AT = 1_000_000;
 // ("999,999 items") and the content is centred within it. A count of a million or more collapses to
 // "1M+", with the exact figure in the title tooltip. Deliberately generic ("item(s)", dynamic plural)
 // so any surface can reuse it for whatever it counts. `stale`: the count is the previous filter's,
-// kept while the new one loads (keepPreviousData) — dimmed and aria-busy, as AdminLogs dims its lines,
-// so a broad search's slow count ("a": ~3 s) never reads as the new result.
-export default function ItemCount({ count, stale = false }: { count?: number; stale?: boolean }) {
+// kept while the new one loads (keepPreviousData) — dimmed, and "updating" for a screen reader (which
+// does not voice aria-busy on plain text), so a broad search's slow count ("a": ~3 s) never reads as
+// the new result. `failed`: the count could not be read — a dash that says so, not the icon-only box
+// of a first load (E31 review 2).
+export default function ItemCount({
+  count,
+  stale = false,
+  failed = false,
+}: {
+  count?: number;
+  stale?: boolean;
+  failed?: boolean;
+}) {
   const { t, i18n } = useTranslation();
   const lang = i18n.language;
-  // `count` is undefined only on a first, un-cached load — keep the fixed box (nothing shifts), just
-  // show the icon until the number arrives.
-  const label =
-    count == null
+  // `count` undefined and not `failed`: a first, un-cached load — keep the fixed box (nothing shifts),
+  // just show the icon until the number arrives.
+  const label = failed
+    ? "—"
+    : count == null
       ? ""
       : t("common.itemCount", {
           count,
@@ -31,7 +42,9 @@ export default function ItemCount({ count, stale = false }: { count?: number; st
   return (
     <span
       data-testid="item-count"
-      title={count == null ? undefined : formatNumber(count, lang)}
+      title={
+        failed ? t("common.countFailed") : count == null ? undefined : formatNumber(count, lang)
+      }
       aria-busy={stale || undefined}
       className="inline-flex h-7 min-w-[9rem] shrink-0 items-center justify-center gap-1.5 rounded-full bg-fg/8 px-3 text-sm whitespace-nowrap text-muted"
     >
@@ -39,6 +52,8 @@ export default function ItemCount({ count, stale = false }: { count?: number; st
       <span className={clsx("tabular-nums transition-opacity", stale && "opacity-60")}>
         {label}
       </span>
+      {failed && <span className="sr-only">{t("common.countFailed")}</span>}
+      {stale && !failed && <span className="sr-only">{t("common.countUpdating")}</span>}
     </span>
   );
 }