feat(keys): j/k and the arrows walk the channel, playlist and download lists

Third and last slice of the E11 keyboard layer (S4c). S4a/S4b covered every card grid and rail; the
table-shaped pages — the channel manager, a playlist, the download center — were still Tab-stop
country: a channel row carries a priority stepper, four action buttons and a tag picker before the next
row's name.

- `ui/DataTable keyboardNav`: the rows are the items, Enter clicks the element a column marks
  `data-nav-open`. DataTable renders its rows twice (a table, and a card list below `md`); both register
  and `lib/listNav` now walks only lists on screen (`checkVisibility`), so `j` past the last row never
  carries on into the hidden copy.
- `lib/listNav` leaves a key something before it already handled (`defaultPrevented`): during a keyboard
  drag a playlist row's grip moves the row with the arrows, and dnd-kit takes the key on the document
  before listNav's window listener. Without the guard ↓ moved the focus off the grip and dropped the drag.
- `ui/NavGrid as="tbody"` for a table whose body is the list (the playlist, under dnd-kit's
  SortableContext).
- Wired: Subscriptions and Discovery tables, the card layout (`ChannelLayoutGrid` → VirtualGrid
  `keyboardNav`), Hidden/Blocked cards (the ChannelLink name is a channel's open element); a playlist's
  rows (the title plays; the grip keeps Space/Enter); the download queue, library and shared lists (the
  play button opens, `IconBtn open`; a row without one opens nothing).
- `listNav.wiring.contract.test`: `VirtualGrid` and `DataTable` must say `keyboardNav` like `Carousel`,
  `DownloadRow` is a card; the audit log's table is the one exemption (its rows open nothing), and an
  exemption that no longer matches a tag fails the test.

Measured in Chrome: Channels table j → row, ring round the whole row; a row walked down keeps 12px above
the viewport edge, walked up clears the sticky header by 35px; Enter opens the channel. Cards 3 columns:
→ 0→1, ↓ 1→4. Hidden and Discovery: j + Enter open the channel. Playlist: j + Enter plays, Escape returns
focus to the row; a keyboard drag (Enter on the grip, ↓) moves the row with focus kept on the grip, and
Escape cancels with the order unchanged. Downloads library: Enter plays a video row, does nothing on a
plain file row.

frontend/src/components/Channels.tsx

@@ -623,6 +623,7 @@ export default function Channels() {
               fixedLayout
               emptyText={t("channels.empty")}
               rowProps={pinMenu.triggerProps}
+              keyboardNav
             />
           ) : (
             <ChannelLayoutGrid