Add frontend pages with login, download, and announcements

- React Router v7: public home page, /login page
- Auth context with JWT localStorage management
- Login: ID/PW form + SSAFY login button (UI only)
- Home: hero banner, download section (login required), announcement board
- API layer with mock data (ready for Go Fiber backend)
- Color scheme: #2E2C2F dark + #BACDB0 accent
- Add .env.example for environment variable reference

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

src/pages/LoginPage.jsx

@@ -0,0 +1,84 @@
+import { useState } from 'react';
+import { useNavigate, Link } from 'react-router-dom';
+import { useAuth } from '../context/AuthContext';
+import './LoginPage.css';
+
+export default function LoginPage() {
+  const [username, setUsername] = useState('');
+  const [password, setPassword] = useState('');
+  const [error, setError] = useState('');
+  const [loading, setLoading] = useState(false);
+  const { login } = useAuth();
+  const navigate = useNavigate();
+
+  const handleSubmit = async (e) => {
+    e.preventDefault();
+    setError('');
+    setLoading(true);
+    try {
+      await login(username, password);
+      navigate('/', { replace: true });
+    } catch (err) {
+      setError(err.message || '로그인에 실패했습니다.');
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <div className="login-page">
+      <div className="login-panel">
+        <div className="login-header">
+          <h1 className="game-title">A301</h1>
+          <p className="game-subtitle">MULTIPLAYER</p>
+        </div>
+
+        <form className="login-form" onSubmit={handleSubmit}>
+          <div className="input-group">
+            <label htmlFor="username">아이디</label>
+            <input
+              id="username"
+              type="text"
+              value={username}
+              onChange={(e) => setUsername(e.target.value)}
+              placeholder="아이디를 입력하세요"
+              autoComplete="username"
+            />
+          </div>
+
+          <div className="input-group">
+            <label htmlFor="password">비밀번호</label>
+            <input
+              id="password"
+              type="password"
+              value={password}
+              onChange={(e) => setPassword(e.target.value)}
+              placeholder="비밀번호를 입력하세요"
+              autoComplete="current-password"
+            />
+          </div>
+
+          {error && <p className="login-error">{error}</p>}
+
+          <button type="submit" className="btn-login" disabled={loading}>
+            {loading ? '로그인 중...' : '로그인'}
+          </button>
+        </form>
+
+        <div className="login-divider">
+          <span>또는</span>
+        </div>
+
+        <button
+          type="button"
+          className="btn-ssafy"
+          onClick={() => alert('SSAFY 로그인은 준비 중입니다.')}
+        >
+          SSAFY 계정으로 로그인
+        </button>
+
+        <Link to="/" className="login-back">메인으로 돌아가기</Link>
+      </div>
+    </div>
+  );
+}