fix(ui): Tabs is an ARIA tab list with roving focus and a wired panel

The rail was a row of aria-pressed toggles: every tab a Tab stop, no arrows,
nothing tying a tab to its content. tablist/tab/aria-selected, one Tab stop,
arrows on either axis, Home/End, and useTabPanel for aria-controls/labelledby.

frontend/src/components/DownloadCenter.confirm.dom.test.tsx

@@ -108,7 +108,7 @@ describe("DownloadCenter confirm dialogs", () => {
     await pick("Clip 1", "downloads.actions.cancel");
     await waitFor(() => expect(mocks.confirm).toHaveBeenCalledTimes(1));
     // The tab's label shares its button with a count badge.
-    fireEvent.click(screen.getByRole("button", { name: /downloads\.tabs\.done/ }));
+    fireEvent.click(screen.getByRole("tab", { name: /downloads\.tabs\.done/ }));
     await screen.findByText("Clip 2");
     await pick("Clip 2", "downloads.actions.redownload");
     await waitFor(() => expect(mocks.confirm).toHaveBeenCalledTimes(2));