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/components/platform/ImpersonationBanner.vue

@@ -0,0 +1,49 @@
+<script setup lang="ts">
+import { useImpersonationStore } from '@/stores/useImpersonationStore'
+
+const impersonationStore = useImpersonationStore()
+
+const isStopping = ref(false)
+
+async function handleStop() {
+  isStopping.value = true
+  await impersonationStore.stopImpersonation()
+}
+</script>
+
+<template>
+  <VBanner
+    v-if="impersonationStore.isImpersonating"
+    color="warning"
+    sticky
+    class="impersonation-banner"
+  >
+    <template #prepend>
+      <VIcon icon="tabler-user-exclamation" />
+    </template>
+
+    <VBannerText>
+      Je bekijkt het platform als
+      <strong>{{ impersonationStore.impersonatedUser?.full_name }}</strong>
+      ({{ impersonationStore.impersonatedUser?.email }})
+    </VBannerText>
+
+    <template #actions>
+      <VBtn
+        variant="tonal"
+        color="warning"
+        :loading="isStopping"
+        prepend-icon="tabler-arrow-back"
+        @click="handleStop"
+      >
+        Terug naar admin
+      </VBtn>
+    </template>
+  </VBanner>
+</template>
+
+<style scoped>
+.impersonation-banner {
+  z-index: 1050;
+}
+</style>