feat(widgets): add frontend widget runtime (types, API, hooks, registry, components)

PR 3 of 4 for configurable dashboard widgets.

- Add TypeScript widget interfaces (WidgetInstance, WidgetInstanceInput,
  WidgetTypeInfo, WidgetDataResponse).
- Create widget API client for CRUD, registry metadata, and per-widget data.
- Create TanStack Query hooks for instances, data, sources, types, and mutations.
- Create closed frontend widget registry with metadata, source type, refresh
  intervals, and config fields.
- Add six shadcn/ui-based widget components: Jellyfin, Backups, Grafana link,
  Prometheus metric, SSH task output, and static text.
- Add Vitest unit tests for registry metadata.

Verification:
- backend ruff clean; pytest 200 passed
- frontend npm run lint: 0 errors
- frontend npm run build: success
- frontend npm run test -- src/widgets/registry.test.ts: 3 passed

frontend/src/widgets/BackupsWidget.tsx

@@ -0,0 +1,66 @@
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import { Badge } from "@/components/ui/badge";
+import { Skeleton } from "@/components/ui/skeleton";
+import { SectionCard } from "../components/SectionCard";
+import { useWidgetData } from "../hooks/useWidgets";
+import type { BackupDashboardSummary } from "../types/backups";
+import type { WidgetInstance } from "../types";
+import { getWidgetDefinition } from "./registry";
+
+interface Props {
+	widget: WidgetInstance;
+}
+
+export function BackupsWidget({ widget }: Props) {
+	const def = getWidgetDefinition(widget.widget_type);
+	const { data, isLoading } = useWidgetData(
+		widget.id,
+		def?.refreshInterval ?? 0,
+	);
+	const summary = data?.data as BackupDashboardSummary | undefined;
+
+	return (
+		<SectionCard title={widget.title} description={def?.description}>
+			{isLoading && !data ? (
+				<div className="flex flex-row flex-wrap gap-6">
+					<Skeleton className="h-10 w-20" />
+					<Skeleton className="h-10 w-20" />
+					<Skeleton className="h-10 w-20" />
+				</div>
+			) : data?.error ? (
+				<Alert variant="destructive">
+					<AlertDescription>{data.error}</AlertDescription>
+				</Alert>
+			) : summary ? (
+				<div className="flex flex-row flex-wrap gap-6">
+					<div>
+						<div className="text-2xl font-semibold">{summary.total_jobs}</div>
+						<div className="text-xs text-muted-foreground">Jobs</div>
+					</div>
+					<div>
+						<div className="text-2xl font-semibold">
+							{summary.success_rate_24h}%
+						</div>
+						<div className="text-xs text-muted-foreground">24h Success</div>
+					</div>
+					<div>
+						<div className="text-2xl font-semibold">
+							{summary.active_alerts > 0 ? (
+								<Badge variant="destructive">{summary.active_alerts}</Badge>
+							) : (
+								0
+							)}
+						</div>
+						<div className="text-xs text-muted-foreground">Alerts</div>
+					</div>
+					{summary.last_failed_at ? (
+						<div className="self-center text-xs text-destructive">
+							Last failed:{" "}
+							{new Date(summary.last_failed_at * 1000).toLocaleString()}
+						</div>
+					) : null}
+				</div>
+			) : null}
+		</SectionCard>
+	);
+}