feat(11-03): responsive shells and storage rows

- App.vue: mobile header with hamburger button; slide-in overlay drawer
  with Teleport backdrop, translate-x-0/-translate-x-full transition;
  drawer state owned by App.vue (D-04/D-05); route-change auto-close
- AdminLayout.vue: matching responsive treatment — hamburger, backdrop,
  drawer, route-change close (RESP-05)
- StorageBrowser.vue: responsive grid templates
  (mobile: grid-cols-[2rem_1fr_6rem], sm: +modified, md: all 5 cols);
  Size hidden below md, Modified hidden below sm; action buttons get
  min-w-[36px] min-h-[36px] touch targets (RESP-02, RESP-03)
- Tests: drawer open/close/backdrop-close/route-change behaviour;
  admin drawer; responsive column and touch-target class assertions

frontend/src/layouts/AdminLayout.vue

@@ -1,7 +1,41 @@
 <template>
   <div class="flex h-screen overflow-hidden">
-    <AdminSidebar />
-    <main class="flex-1 overflow-y-auto">
+    <!-- Mobile-only top header with hamburger (admin) -->
+    <header class="lg:hidden fixed top-0 left-0 right-0 z-30 flex items-center gap-3 px-4 py-3 bg-white border-b border-gray-200">
+      <button
+        @click="drawerOpen = true"
+        aria-label="Open admin navigation"
+        data-test="admin-hamburger-btn"
+        class="p-2 rounded-lg text-gray-500 hover:bg-gray-100 hover:text-gray-700 transition-colors"
+      >
+        <svg xmlns="http://www.w3.org/2000/svg" class="w-5 h-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2">
+          <path stroke-linecap="round" stroke-linejoin="round" d="M4 6h16M4 12h16M4 18h16" />
+        </svg>
+      </button>
+      <span class="text-base font-bold text-indigo-600 tracking-tight">DocuVault</span>
+      <span class="text-xs font-semibold text-indigo-500">Admin</span>
+    </header>
+
+    <!-- Drawer backdrop (mobile only) — teleported to body to guarantee stacking -->
+    <Teleport to="body">
+      <div
+        v-if="drawerOpen"
+        class="lg:hidden fixed inset-0 z-40 bg-black/40"
+        data-test="admin-drawer-backdrop"
+        @click="drawerOpen = false"
+      ></div>
+    </Teleport>
+
+    <!-- Sidebar: always visible on lg+; slides in as overlay drawer below lg -->
+    <div
+      class="fixed inset-y-0 left-0 z-50 transition-transform duration-200 ease-in-out lg:static lg:z-auto lg:translate-x-0 shrink-0"
+      :class="drawerOpen ? 'translate-x-0' : '-translate-x-full'"
+      data-test="admin-sidebar-wrapper"
+    >
+      <AdminSidebar />
+    </div>
+
+    <main class="flex-1 overflow-y-auto pt-[53px] lg:pt-0">
       <div class="p-8 max-w-5xl mx-auto">
         <router-view />
       </div>
@@ -10,5 +44,17 @@
 </template>
 
 <script setup>
+import { ref, watch } from 'vue'
+import { useRoute } from 'vue-router'
 import AdminSidebar from '../components/admin/AdminSidebar.vue'
+
+// Drawer state owned by AdminLayout (per D-04/D-05 — not in AdminSidebar)
+const drawerOpen = ref(false)
+
+const route = useRoute()
+
+// Close drawer on route change (navigation tap closes the drawer automatically)
+watch(() => route.fullPath, () => {
+  drawerOpen.value = false
+})
 </script>