feat(widgets): dashboard loop, widget config UI, and addon pages

PR 4 of 4 for configurable dashboard widgets.

- Replace hard-coded Jellyfin/Backups dashboard sections with a loop that
  renders enabled widget instances by sort_order.
- Add WidgetInstance renderer and WidgetConfigDialog for adding, editing,
  enabling/disabling, deleting, and reordering widgets.
- Add addon pages for grafana, prometheus, and ssh-tasks at /addons/:addonId.
- Register /addons/:addonId route in App.tsx.
- Update docs/REQUIREMENTS.md with the widget system design and API.

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/pages/AddonPage.tsx

@@ -0,0 +1,30 @@
+import { useParams } from "react-router-dom";
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import {
+	GrafanaAddonPage,
+	PrometheusAddonPage,
+	SshTasksAddonPage,
+} from "../addons";
+
+const ADDON_PAGES: Record<string, React.ComponentType> = {
+	grafana: GrafanaAddonPage,
+	prometheus: PrometheusAddonPage,
+	"ssh-tasks": SshTasksAddonPage,
+};
+
+export function AddonPage() {
+	const { addonId } = useParams<{ addonId: string }>();
+	const Page = addonId ? ADDON_PAGES[addonId] : undefined;
+
+	if (!Page) {
+		return (
+			<Alert>
+				<AlertDescription>
+					Addon "{addonId}" is not installed.
+				</AlertDescription>
+			</Alert>
+		);
+	}
+
+	return <Page />;
+}