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/layouts/components/DefaultLayoutWithVerticalNav.vue

@@ -1,7 +1,18 @@
 <script lang="ts" setup>
-import navItems from '@/navigation/vertical'
+import { orgNavItems, platformNavItems } from '@/navigation/vertical'
+import { useAuthStore } from '@/stores/useAuthStore'
+
+const authStore = useAuthStore()
+
+const navItems = computed(() => {
+  if (authStore.isSuperAdmin) {
+    return [...orgNavItems, ...platformNavItems]
+  }
+  return orgNavItems
+})
 
 // Components
+import ImpersonationBanner from '@/components/platform/ImpersonationBanner.vue'
 import Footer from '@/layouts/components/Footer.vue'
 import NavBarNotifications from '@/layouts/components/NavBarNotifications.vue'
 import NavSearchBar from '@/layouts/components/NavSearchBar.vue'
@@ -48,6 +59,9 @@ import { VerticalNavLayout } from '@layouts'
       </div>
     </template>
 
+    <!-- 👉 Impersonation Banner -->
+    <ImpersonationBanner />
+
     <!-- 👉 Pages -->
     <slot />