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/components/WidgetInstance.tsx

@@ -0,0 +1,26 @@
+import { Alert, AlertDescription } from "@/components/ui/alert";
+import { getWidgetDefinition } from "../widgets/registry";
+import type { WidgetInstance } from "../types";
+import { SectionCard } from "./SectionCard";
+
+interface Props {
+	widget: WidgetInstance;
+}
+
+export function WidgetInstance({ widget }: Props) {
+	const def = getWidgetDefinition(widget.widget_type);
+	if (!def) {
+		return (
+			<SectionCard title={widget.title}>
+				<Alert>
+					<AlertDescription>
+						Unknown widget type: {widget.widget_type}
+					</AlertDescription>
+				</Alert>
+			</SectionCard>
+		);
+	}
+
+	const Component = def.component;
+	return <Component widget={widget} />;
+}