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/PlexDiscover.dom.test.tsx

@@ -289,8 +289,9 @@ describe("PlexDiscover · keyboard", () => {
     );
     await screen.findAllByText("Random Pick");
     // Five rails (continue, recently watched, recently added, because-you-watched, surprise) + genres.
+    // Registered by a passive effect, which can land after the cards paint.
+    await waitFor(() => expect(document.querySelectorAll("[data-nav-list]")).toHaveLength(6));
     const lists = [...document.querySelectorAll("[data-nav-list]")];
-    expect(lists.length).toBe(6);
     // …and each holds its cards as items: the genre tiles too, not only the posters.
     expect(lists.every((list) => list.querySelector("[data-nav-index]"))).toBe(true);
     const step = () => {