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/api/auth.js

@@ -0,0 +1,16 @@
+import { apiFetch } from './client';
+
+// TODO: 백엔드 연동 시 mock 제거
+const USE_MOCK = true;
+
+export async function login(username, password) {
+  if (USE_MOCK) {
+    await new Promise((r) => setTimeout(r, 500));
+    if (!username || !password) throw new Error('아이디와 비밀번호를 입력해주세요.');
+    return { token: 'mock-jwt-token', username };
+  }
+  return apiFetch('/api/auth/login', {
+    method: 'POST',
+    body: JSON.stringify({ username, password }),
+  });
+}