fix(#112): remove dead admin breadcrumb code, add logout aria-label

Review feedback: breadcrumb memo had an unused isAdminPage branch
(TopBar no longer renders on admin pages). Added aria-label to
icon-only logout button for screen readers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/AdminLayout.tsx

@@ -34,7 +34,7 @@ export default function AdminLayout() {
         <span className={styles.title}>Admin</span>
         <div className={styles.userSection}>
           <span className={styles.username}>{username}</span>
-          <button className={styles.logoutBtn} onClick={handleLogout}>
+          <button className={styles.logoutBtn} onClick={handleLogout} aria-label="Log out">
             <LogOut size={14} />
           </button>
         </div>