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/PrometheusMetricWidget.tsx

@@ -0,0 +1,61 @@
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import { Skeleton } from "@/components/ui/skeleton";
+import { SectionCard } from "../components/SectionCard";
+import { useWidgetData } from "../hooks/useWidgets";
+import type { WidgetInstance } from "../types";
+import { getWidgetDefinition } from "./registry";
+
+interface Props {
+	widget: WidgetInstance;
+}
+
+type PromQLResult = {
+	resultType?: string;
+	result?: unknown;
+};
+
+type PromQLVectorSample = {
+	metric?: Record<string, string>;
+	value?: [number, string];
+};
+
+function formatPrometheusValue(result: PromQLResult | undefined): string {
+	if (!result) return "No data";
+	if (result.resultType === "scalar" && Array.isArray(result.result)) {
+		return String(result.result[1] ?? "No data");
+	}
+	if (
+		result.resultType === "vector" &&
+		Array.isArray(result.result) &&
+		result.result.length > 0
+	) {
+		const first = result.result[0] as PromQLVectorSample;
+		if (first.value) return String(first.value[1]);
+	}
+	return JSON.stringify(result, null, 2);
+}
+
+export function PrometheusMetricWidget({ widget }: Props) {
+	const def = getWidgetDefinition(widget.widget_type);
+	const { data, isLoading } = useWidgetData(
+		widget.id,
+		def?.refreshInterval ?? 0,
+	);
+	const result = data?.data?.result as PromQLResult | undefined;
+
+	return (
+		<SectionCard title={widget.title} description={def?.description}>
+			{isLoading && !data ? (
+				<Skeleton className="h-10 w-32" />
+			) : data?.error ? (
+				<Alert variant="destructive">
+					<AlertDescription>{data.error}</AlertDescription>
+				</Alert>
+			) : (
+				<pre className="whitespace-pre-wrap text-sm">
+					{formatPrometheusValue(result)}
+				</pre>
+			)}
+		</SectionCard>
+	);
+}