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/__tests__/BackupJobsTable.test.tsx

@@ -0,0 +1,79 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import BackupJobsTable from "../BackupJobsTable";
+import type { BackupJob, BackupRun } from "../../types/backups";
+
+function job(overrides: Partial<BackupJob> = {}): BackupJob {
+	return {
+		id: "j1",
+		name: "nightly",
+		source: "/data",
+		target: "s3://bucket",
+		schedule_interval_seconds: 86400,
+		created_at: 1_700_000_000,
+		...overrides,
+	};
+}
+
+function run(overrides: Partial<BackupRun> = {}): BackupRun {
+	return {
+		id: "r1",
+		job_id: "j1",
+		started_at: 1_700_000_000,
+		ended_at: null,
+		status: "success",
+		bytes_transferred: 2048,
+		duration_ms: 1500,
+		error_message: null,
+		details_json: null,
+		created_at: 1_700_000_000,
+		...overrides,
+	};
+}
+
+// jsdom lacks matchMedia; default to desktop so the table renders.
+function setMatchMedia(matches: boolean) {
+	window.matchMedia = ((query: string) => ({
+		matches: query.includes("768") ? matches : false,
+		media: query,
+		onchange: null,
+		addEventListener: () => {},
+		removeEventListener: () => {},
+		addListener: () => {},
+		removeListener: () => {},
+		dispatchEvent: () => false,
+	})) as unknown as typeof window.matchMedia;
+}
+
+beforeEach(() => setMatchMedia(false));
+
+describe("BackupJobsTable (desktop)", () => {
+	it("renders job name and schedule interval", () => {
+		render(
+			<BackupJobsTable
+				jobs={[job({ name: "nightly", schedule_interval_seconds: 86400 })]}
+				latestRuns={new Map()}
+			/>,
+		);
+		expect(screen.getByText("nightly")).toBeInTheDocument();
+		expect(screen.getByText("1d")).toBeInTheDocument();
+	});
+});
+
+describe("BackupJobsTable (mobile card layout — slice 5)", () => {
+	it("renders cards with job name as primary below md", () => {
+		setMatchMedia(true);
+		render(
+			<BackupJobsTable
+				jobs={[job({ id: "j1", name: "nightly", source: "/data" })]}
+				latestRuns={
+					new Map([["j1", run({ status: "success" })]]) as Map<string, BackupRun>
+				}
+			/>,
+		);
+		expect(screen.getByText("nightly")).toBeInTheDocument();
+		expect(screen.getAllByText("Source")).toHaveLength(1);
+		expect(screen.getAllByText("Schedule")).toHaveLength(1);
+		expect(screen.getAllByText("Last status")).toHaveLength(1);
+	});
+});