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/App.tsx

@@ -22,6 +22,7 @@ import { FileBrowser } from "./pages/FileBrowser";
 import { Actions } from "./pages/Actions";
 import BackupsPage from "./components/BackupsPage";
 import { ObservabilityPage } from "./components/ObservabilityPage";
+import { AddonPage } from "./pages/AddonPage";
 import { getOidcConfig, isOidcConfigured, setAccessToken } from "./auth";
 import { fetchAppVersion } from "./api/client";
 import { FRONTEND_VERSION_LABEL } from "./version";
@@ -449,6 +450,7 @@ function AppInner() {
 								<Route path="/backups" element={<BackupsPage />} />
 								<Route path="/observability" element={<ObservabilityPage />} />
 								<Route path="/settings" element={<Settings />} />
+								<Route path="/addons/:addonId" element={<AddonPage />} />
 							</Route>
 						</Routes>
 					</BrowserRouter>
@@ -480,6 +482,7 @@ function AppInner() {
 							<Route path="/backups" element={<BackupsPage />} />
 							<Route path="/observability" element={<ObservabilityPage />} />
 							<Route path="/settings" element={<Settings />} />
+							<Route path="/addons/:addonId" element={<AddonPage />} />
 						</Route>
 					</Routes>
 				</BrowserRouter>