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.menu.dom.test.tsx

@@ -84,7 +84,7 @@ function mount(tab?: string) {
     </QueryClientProvider>,
   );
   if (tab)
-    fireEvent.click(screen.getByRole("button", { name: new RegExp(`downloads\\.tabs\\.${tab}`) }));
+    fireEvent.click(screen.getByRole("tab", { name: new RegExp(`downloads\\.tabs\\.${tab}`) }));
 }
 
 const rowOf = (text: string) => screen.getByText(text).closest<HTMLElement>("[data-nav-index]")!;