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.dom.test.tsx

@@ -70,4 +70,17 @@ describe("ItemCount", () => {
     expect(screen.getByTestId("item-count").getAttribute("aria-busy")).toBe("true");
     expect(screen.getByText("5 items").className).toContain("opacity-60");
   });
+
+  // E31 review 2: the dim and aria-busy were the only stale cue, and a screen reader voices neither on
+  // plain text; a failed count looked like a first load.
+  it("tells a screen reader a stale count is updating", () => {
+    render(<ItemCount count={5} stale />);
+    expect(text()).toBe("5 itemsupdating");
+  });
+
+  it("says a count that failed, rather than looking like a first load", () => {
+    render(<ItemCount failed />);
+    expect(text()).toBe("—Couldn't count the items");
+    expect(screen.getByTestId("item-count").getAttribute("title")).toBe("Couldn't count the items");
+  });
 });