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/client.js

@@ -0,0 +1,15 @@
+const BASE = import.meta.env.VITE_API_BASE_URL || '';
+
+export async function apiFetch(path, options = {}) {
+  const token = localStorage.getItem('token');
+  const headers = { 'Content-Type': 'application/json', ...options.headers };
+  if (token) headers['Authorization'] = `Bearer ${token}`;
+
+  const res = await fetch(BASE + path, { ...options, headers });
+  if (!res.ok) {
+    const err = new Error(res.statusText);
+    err.status = res.status;
+    throw err;
+  }
+  return res.json();
+}