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/DataTable.progressive.dom.test.tsx

@@ -124,4 +124,32 @@ describe("DataTable — a long page mounts in steps", () => {
     draw({ key: "r90", seq: 1 });
     expect(host.querySelector("tbody")?.textContent).toContain("r 90");
   });
+
+  // E31 review 2: the reveal floor held for the page's lifetime. Channels never clears its `follow`, so
+  // after one priority pick every later sort or search mounted every row up to that channel at once.
+  it("lets a revealed row go once the reader moves to another view", () => {
+    host = document.createElement("div");
+    document.body.appendChild(host);
+    root = createRoot(host);
+    const rows = many(200);
+    const draw = (reveal: { key: string; seq: number } | null, narrowedBy: string) =>
+      flushSync(() =>
+        root!.render(
+          <DataTable
+            rows={rows}
+            columns={plain}
+            rowKey={(x) => x.id}
+            pageSize={0}
+            reveal={reveal}
+            narrowedBy={narrowedBy}
+          />,
+        ),
+      );
+    draw(null, "");
+    const follow = { key: "r150", seq: 1 };
+    draw(follow, "");
+    expect(host.querySelector("tbody")?.textContent).toContain("r 150");
+    draw(follow, "s"); // a search: a new view, the same (never cleared) reveal
+    expect(bodyRows()).toBe(FIRST_ROWS);
+  });
 });