refactor(admin): move sections to dynamic-route pattern + reduce overview

Each admin section component now accepts an optional `id` prop (was
required) so it works under the new /admin/[section] dynamic route
without scroll-anchor wrapping. Adds <IntegrationsGroupSection> as the
composite for /admin/integrations (Umami + GlitchTip + WebPush + Bug
Report). Reduces /admin to a true overview: status-card grid + welcome
card + quick-jump menu of sub-sections; the long monolithic scroll is
gone.

Bundle-size win: each section now ships under its own route bundle, so
the overview no longer drags Umami/GlitchTip/Feedback/Users/etc. trees.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/admin/api-token-overview-section.tsx

@@ -9,7 +9,7 @@ import { formatDate, formatDateTime } from "@/lib/format";
 import { useTranslations } from "@/lib/i18n/context";
 import { type ApiTokenInfo } from "./_shared";
 
-export function ApiTokenOverviewSection({ id }: { id: string }) {
+export function ApiTokenOverviewSection({ id }: { id?: string } = {}) {
   const { t } = useTranslations();
   const [expanded, setExpanded] = useState(false);