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>

package.json

@@ -11,7 +11,8 @@
   },
   "dependencies": {
     "react": "^19.2.0",
-    "react-dom": "^19.2.0"
+    "react-dom": "^19.2.0",
+    "react-router-dom": "^7.13.1"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",