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/JellyfinWidget.tsx

@@ -0,0 +1,40 @@
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import { Skeleton } from "@/components/ui/skeleton";
+import { SessionActivityPanel } from "../components/SessionActivityPanel";
+import { SectionCard } from "../components/SectionCard";
+import { useWidgetData } from "../hooks/useWidgets";
+import type { NowPlayingSession, WidgetInstance } from "../types";
+import { getWidgetDefinition } from "./registry";
+
+interface Props {
+	widget: WidgetInstance;
+}
+
+export function JellyfinWidget({ widget }: Props) {
+	const def = getWidgetDefinition(widget.widget_type);
+	const { data, isLoading } = useWidgetData(
+		widget.id,
+		def?.refreshInterval ?? 0,
+	);
+	const sessions = data?.data?.sessions as NowPlayingSession[] | undefined;
+
+	return (
+		<SectionCard title={widget.title} description={def?.description}>
+			{isLoading && !data ? (
+				<div className="flex flex-col gap-2">
+					<Skeleton className="h-4 w-3/4" />
+					<Skeleton className="h-4 w-1/2" />
+				</div>
+			) : data?.error ? (
+				<Alert variant="destructive">
+					<AlertDescription>{data.error}</AlertDescription>
+				</Alert>
+			) : Array.isArray(sessions) ? (
+				<SessionActivityPanel
+					sessions={sessions}
+					emptyMessage="No recent user activity sessions right now."
+				/>
+			) : null}
+		</SectionCard>
+	);
+}