admin: fix dashboard phantom scroll, drop last-failure column from upstream table

the chart's screen-reader table wore srOnly directly; overflow and
height do not apply to a table box, so it laid out 1200px tall below
the page while clip-path hid the paint. wrap it in a hidden div, which
clips properly and keeps the table role. failure detail is the
diagnostics page's job since milestone 27; the column and the now
dead formatAge go.

admin/src/features/dashboard/DashboardPage.test.tsx

@@ -193,7 +193,6 @@ test("dashboard renders stats, chart, disk card, upstream table and health banne
 	expect(screen.getByText("https://dns.example/dns-query")).toBeTruthy();
 	expect(screen.getByText("90.0%")).toBeTruthy();
 	expect(screen.getByText("100.0%")).toBeTruthy();
-	expect(screen.getByText("timeout · 3h ago")).toBeTruthy();
 	expect(screen.getByText("1/2 available")).toBeTruthy();
 });