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/BackupRunsTable.tsx

@@ -15,6 +15,11 @@ import {
 	TableHeader,
 	TableRow,
 } from "@/components/ui/table";
+import {
+	MobileCardRow,
+	type MobileCardField,
+} from "@/components/ui/mobile-card";
+import { useIsMobile } from "../hooks/useIsMobile";
 import type { BackupRun } from "../types/backups";
 
 interface Props {
@@ -55,8 +60,35 @@ function statusVariant(status: string): StatusVariant {
 	return "warning";
 }
 
+// Mobile card fields (spec R3.2): job_id is primary; status/duration/size/
+// started give the at-a-glance info. See OpenSpec change `mobile-responsive-parity`.
+const runCardFields: MobileCardField<BackupRun>[] = [
+	{ key: "job", label: "Job", render: (r) => r.job_id, primary: true },
+	{
+		key: "status",
+		label: "Status",
+		render: (r) => <Badge variant={statusVariant(r.status)}>{r.status}</Badge>,
+	},
+	{
+		key: "duration",
+		label: "Duration",
+		render: (r) => formatDuration(r.duration_ms),
+	},
+	{
+		key: "size",
+		label: "Size",
+		render: (r) => formatBytes(r.bytes_transferred),
+	},
+	{
+		key: "started",
+		label: "Started",
+		render: (r) => formatTimestamp(r.started_at),
+	},
+];
+
 export default function BackupRunsTable({ runs }: Props) {
 	const [statusFilter, setStatusFilter] = useState<string>("all");
+	const isMobile = useIsMobile();
 
 	const filteredRuns =
 		statusFilter === "all"
@@ -77,34 +109,42 @@ export default function BackupRunsTable({ runs }: Props) {
 				</SelectContent>
 			</Select>
 
-			<div className="overflow-hidden rounded-lg border border-border">
-				<Table aria-label="Backup runs">
-					<TableHeader>
-						<TableRow className="bg-card hover:bg-card">
-							<TableHead>Job</TableHead>
-							<TableHead>Status</TableHead>
-							<TableHead>Duration</TableHead>
-							<TableHead>Size</TableHead>
-							<TableHead>Started</TableHead>
-						</TableRow>
-					</TableHeader>
-					<TableBody>
-						{filteredRuns.map((run) => (
-							<TableRow key={run.id}>
-								<TableCell>{run.job_id}</TableCell>
-								<TableCell>
-									<Badge variant={statusVariant(run.status)}>
-										{run.status}
-									</Badge>
-								</TableCell>
-								<TableCell>{formatDuration(run.duration_ms)}</TableCell>
-								<TableCell>{formatBytes(run.bytes_transferred)}</TableCell>
-								<TableCell>{formatTimestamp(run.started_at)}</TableCell>
+			{isMobile ? (
+				<MobileCardRow
+					rows={filteredRuns}
+					fields={runCardFields}
+					getRowId={(r) => r.id}
+				/>
+			) : (
+				<div className="overflow-hidden rounded-lg border border-border">
+					<Table aria-label="Backup runs">
+						<TableHeader>
+							<TableRow className="bg-card hover:bg-card">
+								<TableHead>Job</TableHead>
+								<TableHead>Status</TableHead>
+								<TableHead>Duration</TableHead>
+								<TableHead>Size</TableHead>
+								<TableHead>Started</TableHead>
 							</TableRow>
-						))}
-					</TableBody>
-				</Table>
-			</div>
+						</TableHeader>
+						<TableBody>
+							{filteredRuns.map((run) => (
+								<TableRow key={run.id}>
+									<TableCell>{run.job_id}</TableCell>
+									<TableCell>
+										<Badge variant={statusVariant(run.status)}>
+											{run.status}
+										</Badge>
+									</TableCell>
+									<TableCell>{formatDuration(run.duration_ms)}</TableCell>
+									<TableCell>{formatBytes(run.bytes_transferred)}</TableCell>
+									<TableCell>{formatTimestamp(run.started_at)}</TableCell>
+								</TableRow>
+							))}
+						</TableBody>
+					</Table>
+				</div>
+			)}
 		</div>
 	);
 }