Mobile Users + Backups tables: stacked cards + selection (Slice 5)

Below md, the Users directory and the three Backups tables render as
MobileCardRow cards:

- UsersPage: display name primary; username/activity/email fields. Each
  card carries a selection checkbox (44px via mobile-touch-target) in the
  actions slot with stopPropagation so toggling selection does not open
  the drawer; card-body tap still opens the drawer.
- BackupAlertsTable: alert message primary; severity/type/created fields;
  Acknowledge action preserved in actions slot.
- BackupJobsTable: job name primary; source/schedule/last-status fields
  (joins latestRuns into a JobCardRow).
- BackupRunsTable: run job_id primary; status/duration/size/started fields;
  status-filter Select renders above both layouts (preserved on mobile).

Desktop (md+) is byte-for-byte identical for all four components -- the
UsersPage diff is dominated by re-indenting the existing Table into the
isMobile ternary else branch.

Fix from Slice 5 review: MobileCardRow now renders the clickable card as
<div role=button tabIndex=0> with Enter/Space keyboard handling instead
of <button>, so nesting a Radix Checkbox (which renders a <button>) in
the actions slot produces valid HTML. The desktop-parity argument for
<button>-in-<button> did not hold (desktop rows are <tr>, not buttons).

Cross-cutting: useIsMobile hardened with typeof window.matchMedia guard
(safe in real browsers; only changes jsdom crash -> false). The file-local
900px compose hook was renamed useComposeViewport to avoid collision with
the shared 768px useIsMobile.

Tests: BackupJobsTable test file added (was untested), UsersPage mobile
selection round-trip + stopPropagation, mobile card render across all
four components. 105 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/ (spec R3, tasks slice 5).

frontend/src/hooks/useIsMobile.ts

@@ -17,11 +17,17 @@ const MOBILE_QUERY = "(max-width: 768px)";
 export function useIsMobile(): boolean {
 	const [isMobile, setIsMobile] = useState(
 		() =>
-			typeof window !== "undefined" && window.matchMedia(MOBILE_QUERY).matches,
+			typeof window !== "undefined" &&
+			typeof window.matchMedia === "function" &&
+			window.matchMedia(MOBILE_QUERY).matches,
 	);
 
 	useEffect(() => {
-		if (typeof window === "undefined") return;
+		if (
+			typeof window === "undefined" ||
+			typeof window.matchMedia !== "function"
+		)
+			return;
 		const mql = window.matchMedia(MOBILE_QUERY);
 		const handler = (e: MediaQueryListEvent) => setIsMobile(e.matches);
 		mql.addEventListener("change", handler);