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/Tabs.tsx

@@ -8,9 +8,11 @@ import CountBadge from "./CountBadge";
 // the Settings-style side rail (full-width, left-aligned, icon + label) — from the `sm` breakpoint up.
 // Below it a side rail leaves a phone too little width for the content beside it (measured at 390 px:
 // the rail clipped its own labels and Settings → Account pushed its buttons over their text), so there
-// it is a horizontal row that scrolls. Callers persist the active tab with lib/storage's
-// useTabPersistedState (per browser tab, E31 S4b): a reload keeps the user where they were, and a
-// fresh tab opens on the first sub-tab rather than wherever the last visit ended.
+// it is a horizontal row that scrolls. A page whose sub-tab is worth keeping persists it with
+// lib/storage's useTabPersistedState (per browser tab, E31 S4b: Admin Users, Config, Settings, Stats):
+// a reload keeps the user where they were, and a fresh tab opens on the first sub-tab. Not every
+// caller: the channel manager's view is account-wide (ChannelsProvider), and Downloads, a channel page
+// and the player settings start on their first tab each time.
 //
 // An ARIA tab list (E28 review F3), not a row of toggle buttons: `tablist` / `tab` / `aria-selected`,
 // ONE Tab stop (the selected tab; roving tabindex), the arrows move and select (either axis, since the