feat(09-02): create AdminLayout.vue and AdminSidebar.vue

- AdminLayout: flex h-screen shell with AdminSidebar + main content area (p-8 max-w-5xl mx-auto router-view)
- AdminSidebar: 5 nav links in D-07 order (Overview/Users/Quotas/AI Config/Audit Log) with SVG icons
- Admin badge subtitle ("Admin" in indigo-500 per D-05) replaces "Document Manager"
- No Back-to-app link (D-06 applied)
- User identity footer + sign-out reused verbatim from AppSidebar.vue pattern
- Scoped nav-link/nav-link-active CSS identical to AppSidebar.vue

frontend/src/layouts/AdminLayout.vue

@@ -0,0 +1,14 @@
+<template>
+  <div class="flex h-screen overflow-hidden">
+    <AdminSidebar />
+    <main class="flex-1 overflow-y-auto">
+      <div class="p-8 max-w-5xl mx-auto">
+        <router-view />
+      </div>
+    </main>
+  </div>
+</template>
+
+<script setup>
+import AdminSidebar from '../components/admin/AdminSidebar.vue'
+</script>