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/BackupJobsTable.tsx

@@ -7,6 +7,11 @@ import {
 	TableHeader,
 	TableRow,
 } from "@/components/ui/table";
+import {
+	MobileCardRow,
+	type MobileCardField,
+} from "@/components/ui/mobile-card";
+import { useIsMobile } from "../hooks/useIsMobile";
 import type { BackupJob, BackupRun } from "../types/backups";
 
 interface Props {
@@ -41,7 +46,54 @@ function statusVariant(status: string): StatusVariant {
 	return "secondary";
 }
 
+// Mobile card fields (spec R3.2): job name is primary; source/schedule/status
+// give at-a-glance context. See OpenSpec change `mobile-responsive-parity`.
+interface JobCardRow {
+	job: BackupJob;
+	status: string;
+	run_started: number | null;
+}
+
+const jobCardFields: MobileCardField<JobCardRow>[] = [
+	{ key: "name", label: "Name", render: (r) => r.job.name, primary: true },
+	{
+		key: "source",
+		label: "Source",
+		render: (r) => r.job.source ?? "—",
+	},
+	{
+		key: "schedule",
+		label: "Schedule",
+		render: (r) => formatInterval(r.job.schedule_interval_seconds),
+	},
+	{
+		key: "status",
+		label: "Last status",
+		render: (r) => <Badge variant={statusVariant(r.status)}>{r.status}</Badge>,
+	},
+];
+
 export default function BackupJobsTable({ jobs, latestRuns }: Props) {
+	const isMobile = useIsMobile();
+
+	if (isMobile) {
+		const cardRows: JobCardRow[] = jobs.map((job) => {
+			const run = latestRuns.get(job.id);
+			return {
+				job,
+				status: run?.status ?? "unknown",
+				run_started: run?.started_at ?? null,
+			};
+		});
+		return (
+			<MobileCardRow
+				rows={cardRows}
+				fields={jobCardFields}
+				getRowId={(r) => r.job.id}
+			/>
+		);
+	}
+
 	return (
 		<div className="overflow-hidden rounded-lg border border-border">
 			<Table aria-label="Backup jobs">