test: a list key waits for the list's registration, not only its rows

The server gate's frontend job went red on 0.96.0: PlexPlaylistView's j/Delete
test pressed j after the rows painted but before useListNav's passive effect
registered the list, so the key did nothing on the slower runner. Every test
that presses a list key after an async load now waits for [data-nav-list] on
its list first (PlexPlaylistView, Playlists, Home, PlexDiscover). Checked
against a 30 ms delayed registration: the old tests fail, the new ones pass.

frontend/src/components/Home.dom.test.tsx

@@ -796,7 +796,9 @@ describe("Home · keyboard", () => {
       ],
     });
     mount();
-    await screen.findByText("Comfort thing");
+    // The rail's keys are live once its effect registers it, which can land after the cards paint.
+    const first = await screen.findByText("Comfort thing");
+    await waitFor(() => expect(first.closest("[data-nav-list]")).not.toBeNull());
     const titles = ["Comfort thing", "Pinned channel", "Second thing", "Third thing"];
     const seen: (string | undefined)[] = [];
     for (let i = 0; i < titles.length; i++) {