feat(#112): add admin header bar with back button and logout

AdminLayout gains a self-contained header (Back / Admin / user+logout)
with CSS module styles, replacing the inline padding wrapper. Admin
pages now render fully without the main app chrome.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/pages/Admin/AdminLayout.tsx

@@ -1,5 +1,8 @@
 import { Outlet, useNavigate, useLocation } from 'react-router';
 import { Tabs } from '@cameleer/design-system';
+import { ArrowLeft, LogOut } from 'lucide-react';
+import { useAuthStore } from '../../auth/auth-store';
+import styles from './AdminLayout.module.css';
 
 const ADMIN_TABS = [
   { label: 'User Management', value: '/admin/rbac' },
@@ -13,15 +16,35 @@ const ADMIN_TABS = [
 export default function AdminLayout() {
   const navigate = useNavigate();
   const location = useLocation();
+  const { username, logout } = useAuthStore();
+
+  const handleBack = () => navigate('/exchanges');
+  const handleLogout = () => {
+    logout();
+    navigate('/login');
+  };
 
   return (
     <div>
+      <header className={styles.header}>
+        <button className={styles.backBtn} onClick={handleBack}>
+          <ArrowLeft size={16} />
+          Back
+        </button>
+        <span className={styles.title}>Admin</span>
+        <div className={styles.userSection}>
+          <span className={styles.username}>{username}</span>
+          <button className={styles.logoutBtn} onClick={handleLogout}>
+            <LogOut size={14} />
+          </button>
+        </div>
+      </header>
       <Tabs
         tabs={ADMIN_TABS}
         active={location.pathname}
         onChange={(path) => navigate(path)}
       />
-      <div style={{ padding: '20px 24px 40px' }}>
+      <div className={styles.content}>
         <Outlet />
       </div>
     </div>