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/components.d.ts

@@ -64,6 +64,7 @@ declare module 'vue' {
     EventMetricCards: typeof import('./src/components/events/EventMetricCards.vue')['default']
     EventTabsNav: typeof import('./src/components/events/EventTabsNav.vue')['default']
     I18n: typeof import('./src/@core/components/I18n.vue')['default']
+    ImpersonationBanner: typeof import('./src/components/platform/ImpersonationBanner.vue')['default']
     ImportFromEventDialog: typeof import('./src/components/event/ImportFromEventDialog.vue')['default']
     InfoTooltip: typeof import('./src/components/common/InfoTooltip.vue')['default']
     InviteMemberDialog: typeof import('./src/components/members/InviteMemberDialog.vue')['default']