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/HomePage.jsx

@@ -0,0 +1,39 @@
+import { Link } from 'react-router-dom';
+import { useAuth } from '../context/AuthContext';
+import DownloadSection from '../components/DownloadSection';
+import AnnouncementBoard from '../components/AnnouncementBoard';
+import './HomePage.css';
+
+export default function HomePage() {
+  const { user, logout } = useAuth();
+
+  return (
+    <div className="home-page">
+      <header className="home-header">
+        <h1 className="home-logo">A301</h1>
+        <div className="home-user">
+          {user ? (
+            <>
+              <span className="home-username">{user.username}</span>
+              <button className="btn-logout" onClick={logout}>로그아웃</button>
+            </>
+          ) : (
+            <Link to="/login" className="btn-header-login">로그인</Link>
+          )}
+        </div>
+      </header>
+
+      <section className="hero-banner">
+        <div className="hero-overlay">
+          <h2 className="hero-title">A301 MULTIPLAYER</h2>
+          <p className="hero-desc">Unity 3D 멀티플레이어 테스트에 참여하세요</p>
+        </div>
+      </section>
+
+      <main className="home-main">
+        <DownloadSection />
+        <AnnouncementBoard />
+      </main>
+    </div>
+  );
+}