Fix hardcoded localhost:8000 URLs

- Add backend/utils.py for URL management
- Update serializers to use normalize_file_url()
- Update views to use URL utils from env vars
- Fix frontend components to use NEXT_PUBLIC_API_URL
- Add new env vars: DJANGO_BACKEND_URL, DJANGO_MEDIA_BASE_URL
- Replace all hardcoded localhost:8000 with configurable URLs

frontend/linktree-frontend/src/app/auth/register/page.tsx

@@ -46,7 +46,7 @@ export default function RegisterPage() {
     }
 
     try {
-      const API = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000'
+      const API = process.env.NEXT_PUBLIC_API_URL || 'https://links.shareon.kr'
       const response = await fetch(`${API}/api/auth/register/`, {
         method: 'POST',
         headers: {