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/addons/GrafanaAddonPage.tsx

@@ -0,0 +1,37 @@
+import { ExternalLink } from "lucide-react";
+import { Button } from "@/components/ui/button";
+import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
+
+export function GrafanaAddonPage() {
+	const grafanaUrl =
+		(import.meta.env.VITE_GRAFANA_URL as string | undefined) ||
+		"http://localhost:3000";
+
+	return (
+		<div className="flex flex-col gap-4">
+			<h2 className="text-xl font-semibold">Grafana</h2>
+			<Card>
+				<CardHeader>
+					<CardTitle>Metrics & logs</CardTitle>
+				</CardHeader>
+				<CardContent className="flex flex-col gap-3">
+					<p className="text-sm text-muted-foreground">
+						Open the full Grafana instance for dashboards, metrics, and
+						log exploration.
+					</p>
+					<Button asChild>
+						<a
+							href={grafanaUrl}
+							target="_blank"
+							rel="noopener noreferrer"
+							className="inline-flex items-center"
+						>
+							Open Grafana
+							<ExternalLink className="ml-2 h-4 w-4" />
+						</a>
+					</Button>
+				</CardContent>
+			</Card>
+		</div>
+	);
+}