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/components/ui/mobile-card.tsx

@@ -91,14 +91,21 @@ export function MobileCardRow<T>({
 
 				if (onRowClick) {
 					return (
-						<button
+						<div
 							key={rowKey}
-							type="button"
+							role="button"
+							tabIndex={0}
 							onClick={() => onRowClick(row)}
-							className="mobile-touch-target min-h-11 w-full rounded-lg border border-border bg-card p-3 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring"
+							onKeyDown={(e) => {
+								if (e.key === "Enter" || e.key === " ") {
+									e.preventDefault();
+									onRowClick(row);
+								}
+							}}
+							className="mobile-touch-target min-h-11 w-full cursor-pointer rounded-lg border border-border bg-card p-3 text-left transition-colors hover:bg-muted/50 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring"
 						>
 							{body}
-						</button>
+						</div>
 					);
 				}