feat(06.2-05): show @handle in AccountView and AdminUsersTab

- Add Username row (@handle) to Account information section in AccountView.vue
- Add Handle column (th + td with @prefix) to users table in AdminUsersTab.vue
- Both use existing data already present in API responses (no backend changes)

frontend/src/components/admin/AdminUsersTab.vue

@@ -112,6 +112,7 @@
         <thead>
           <tr class="bg-gray-50 text-left">
             <th class="px-4 py-3 text-xs font-semibold text-gray-500 uppercase tracking-wider">Email</th>
+            <th class="px-4 py-3 text-xs font-semibold text-gray-500 uppercase tracking-wider">Handle</th>
             <th class="px-4 py-3 text-xs font-semibold text-gray-500 uppercase tracking-wider">Role</th>
             <th class="px-4 py-3 text-xs font-semibold text-gray-500 uppercase tracking-wider">Status</th>
             <th class="px-4 py-3 text-xs font-semibold text-gray-500 uppercase tracking-wider">Created</th>
@@ -129,6 +130,7 @@
             ]"
           >
             <td class="px-4 py-3 text-gray-900">{{ user.email }}</td>
+            <td class="px-4 py-3 text-gray-500 font-mono text-xs">{{ user.handle ? '@' + user.handle : '—' }}</td>
             <td class="px-4 py-3">
               <span
                 class="inline-flex items-center px-2 py-1 rounded text-xs font-semibold"

frontend/src/views/AccountView.vue

@@ -9,6 +9,7 @@
         <h3 class="font-semibold text-gray-800 mb-4">Account information</h3>
         <div class="space-y-2 text-sm text-gray-700">
           <div><span class="text-gray-500">Email:</span> {{ authStore.user?.email }}</div>
+          <div><span class="text-gray-500">Username:</span> @{{ authStore.user?.handle }}</div>
           <div class="flex items-center gap-2">
             <span class="text-gray-500">Role:</span>
             <span