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/downloadRow.dom.test.tsx

@@ -12,7 +12,7 @@ vi.mock("react-i18next", async (importOriginal) => {
   return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
 });
 
-import { DownloadRow } from "./downloadRow";
+import { DownloadRow, IconBtn } from "./downloadRow";
 import type { DownloadJob } from "../lib/api";
 
 const baseJob: DownloadJob = {
@@ -116,3 +116,27 @@ describe("DownloadRow file kind", () => {
     expect(container.querySelector("img")).not.toBeNull();
   });
 });
+
+describe("DownloadRow · keyboard", () => {
+  it("is a list item only when it is given a position", () => {
+    const { container, rerender } = render(<DownloadRow job={baseJob} navIndex={3} />);
+    expect(container.firstElementChild?.getAttribute("data-nav-index")).toBe("3");
+    rerender(<DownloadRow job={baseJob} />);
+    expect(container.firstElementChild?.hasAttribute("data-nav-index")).toBe(false);
+  });
+
+  it("marks the action that opens the row, and only that one", () => {
+    render(
+      <DownloadRow job={baseJob} navIndex={0}>
+        <IconBtn open onClick={() => {}} title="play">
+          p
+        </IconBtn>
+        <IconBtn onClick={() => {}} title="delete">
+          d
+        </IconBtn>
+      </DownloadRow>,
+    );
+    expect(screen.getByTitle("play").hasAttribute("data-nav-open")).toBe(true);
+    expect(screen.getByTitle("delete").hasAttribute("data-nav-open")).toBe(false);
+  });
+});