hoàn thành chức năng hiện username annotation của từng user

frontend/src/app/admin/page.tsx

@@ -151,6 +151,15 @@ export default function AdminPage() {
           </div>
           <div className="flex items-center gap-3">
             <span className="text-sm text-gray-500 hidden sm:block">{me?.username}</span>
+            <button
+              onClick={() => router.push("/change-password")}
+              className="text-sm text-gray-500 hover:text-blue-600 transition"
+              title="Đổi mật khẩu"
+            >
+              <svg className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 7a2 2 0 012 2m4 0a6 6 0 01-7.743 5.743L11 17H9v2H7v2H4a1 1 0 01-1-1v-2.586a1 1 0 01.293-.707l5.964-5.964A6 6 0 1121 9z" />
+              </svg>
+            </button>
             <button
               onClick={async () => { await api.logout(); router.replace("/login"); }}
               className="text-sm text-gray-500 hover:text-red-600 transition"