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/components/LayoutWrapper.tsx

@@ -86,7 +86,7 @@ export function LayoutWrapper({ children }: { children: ReactNode }) {
                       user.avatar && user.avatar.startsWith('http')
                         ? user.avatar
                         : user.avatar 
-                          ? `http://localhost:8000${user.avatar}`
+                          ? `${process.env.NEXT_PUBLIC_API_URL || 'https://links.shareon.kr'}${user.avatar}`
                           : '/assets/img/avatar-dhg.png'
                     }
                     alt="Avatar"