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/ui/styles.ts

@@ -193,7 +193,15 @@ export const styles = stylex.create({
 	mono: {
 		fontFamily: "ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace",
 	},
-	/** Visible to a screen reader only; the element keeps its place in the a11y tree. */
+	/**
+	 * Visible to a screen reader only; the element keeps its place in the a11y tree.
+	 *
+	 * Apply it to a block container. `overflow` has no effect on a table box, and
+	 * `height` on one is a minimum, so a `<table>` wearing this still lays out at
+	 * its full content height and pushes the page's scrollable overflow past the
+	 * app shell — invisible, because `clip-path` still hides the paint. Wrap the
+	 * table in a hidden `<div>` instead of hiding the table itself.
+	 */
 	srOnly: {
 		position: "absolute",
 		width: 1,