feat: platform admin frontend — pages, composables, navigation, impersonation

Build the frontend for platform admin in apps/app/:
- TypeScript types (admin.ts) and API composable (useAdmin.ts) with
  TanStack Query for all admin endpoints
- ImpersonationStore (Pinia) + ImpersonationBanner component integrated
  in the main layout, with token-based session management
- Platform navigation section (conditionally shown for super_admin users)
- Route guard blocking /platform/* for non-super_admin users
- 6 pages: dashboard with stats cards, organisations list/detail,
  users list/detail with impersonation, activity log with expandable rows
- All pages implement loading/error/empty states per conventions
- Vite build passes cleanly

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/app/src/navigation/vertical/index.ts

@@ -1,4 +1,4 @@
-export default [
+export const orgNavItems = [
   {
     title: 'Dashboard',
     to: { name: 'dashboard' },
@@ -35,3 +35,31 @@ export default [
     icon: { icon: 'tabler-settings' },
   },
 ]
+
+export const platformNavItems = [
+  {
+    heading: 'Platform',
+  },
+  {
+    title: 'Platform Dashboard',
+    to: { name: 'platform' },
+    icon: { icon: 'tabler-chart-dots-3' },
+  },
+  {
+    title: 'Organisaties',
+    to: { name: 'platform-organisations' },
+    icon: { icon: 'tabler-buildings' },
+  },
+  {
+    title: 'Gebruikers',
+    to: { name: 'platform-users' },
+    icon: { icon: 'tabler-users-group' },
+  },
+  {
+    title: 'Activity Log',
+    to: { name: 'platform-activity-log' },
+    icon: { icon: 'tabler-list-details' },
+  },
+]
+
+export default orgNavItems