style: 헤더 로그인 버튼을 LogIn 아이콘으로 변경

- 텍스트 '로그인' 대신 lucide-react LogIn 아이콘 사용
- 프로필 버튼과 동일한 크기(9x9)로 일관성 유지

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: hchang

moducon-frontend/src/components/layout/Header.tsx

@@ -4,7 +4,7 @@ import Link from 'next/link';
 import Image from 'next/image';
 import { usePathname } from 'next/navigation';
 import { useAuthStore } from '@/store/authStore';
-import { Home } from 'lucide-react';
+import { Home, LogIn } from 'lucide-react';
 
 export function Header() {
   const pathname = usePathname();
@@ -55,9 +55,9 @@ export function Header() {
         ) : (
           <Link
             href="/login"
-            className="px-3 py-1.5 rounded-full bg-white/20 hover:bg-white/30 text-white text-sm font-medium transition-colors border border-white/50"
+            className="w-9 h-9 rounded-full bg-white/20 hover:bg-white/30 flex items-center justify-center transition-colors border border-white/50"
           >
-            로그인
+            <LogIn className="w-5 h-5 text-white" />
           </Link>
         )}
       </div>