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/index.ts

@@ -0,0 +1,8 @@
+export { BackupsWidget } from "./BackupsWidget";
+export { GrafanaLinkWidget } from "./GrafanaLinkWidget";
+export { JellyfinWidget } from "./JellyfinWidget";
+export { PrometheusMetricWidget } from "./PrometheusMetricWidget";
+export { SshTaskWidget } from "./SshTaskWidget";
+export { StaticWidget } from "./StaticWidget";
+export { getWidgetDefinition, listWidgetTypes, WIDGET_REGISTRY } from "./registry";
+export type { WidgetConfigField, WidgetDefinition } from "./registry";