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/plugins/1.router/guards.ts

@@ -44,6 +44,17 @@ export function setupGuards(router: Router) {
       return { path: '/login', query: { to: to.fullPath } }
     }
 
+    // Platform admin routes — require super_admin role
+    if (to.path.startsWith('/platform')) {
+      if (!authStore.isSuperAdmin) {
+        if (import.meta.env.DEV) console.log('🚫 Not a super admin, redirecting to dashboard')
+        return { name: 'dashboard' }
+      }
+      // Platform routes don't require organisation selection
+      if (import.meta.env.DEV) console.log('✅ Super admin access to platform route')
+      return
+    }
+
     // Authenticated — check organisation selection for routes that need it
     const orgStore = useOrganisationStore()
     const isSelectOrgPage = to.path === '/select-organisation'