feat: implement initial login UI and entry logic

src/main.ts

@@ -136,4 +136,36 @@ function initApp() {
   window.addEventListener('refresh-view', () => refreshView());
 }
 
-document.addEventListener('DOMContentLoaded', initApp);
+/**
+ * 로그인 처리 로직
+ */
+function handleLogin() {
+  const loginContainer = document.getElementById('login-container');
+  const appLayout = document.getElementById('app-layout');
+  const loginForm = document.getElementById('login-form') as HTMLFormElement;
+
+  if (!loginForm || !loginContainer || !appLayout) return;
+
+  loginForm.addEventListener('submit', (e) => {
+    e.preventDefault();
+    
+    // Mock 로그인: 아이디/비밀번호 입력 여부만 확인
+    const username = (document.getElementById('username') as HTMLInputElement).value;
+    const password = (document.getElementById('password') as HTMLInputElement).value;
+
+    if (username && password) {
+      console.log('🔓 Login successful (Mock)');
+      
+      // UI 전환
+      loginContainer.style.display = 'none';
+      appLayout.style.display = 'flex';
+      
+      // 앱 초기화 시작
+      initApp();
+    } else {
+      alert('아이디와 비밀번호를 입력해주세요.');
+    }
+  });
+}
+
+document.addEventListener('DOMContentLoaded', handleLogin);