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/CustomizationPanel.tsx

@@ -55,7 +55,7 @@ export function CustomizationPanel({ isOpen, onClose, onSettingsUpdate }: Custom
   const loadSettings = async () => {
     try {
       const token = localStorage.getItem('token')
-      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/customization/settings/`, {
         headers: {
           'Authorization': `Bearer ${token}`
@@ -75,7 +75,7 @@ export function CustomizationPanel({ isOpen, onClose, onSettingsUpdate }: Custom
     setLoading(true)
     try {
       const token = localStorage.getItem('token')
-      const API = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000'
+      const API = process.env.NEXT_PUBLIC_API_URL || 'https://links.shareon.kr'
       
       // Если есть новый файл фоновой картинки, отправляем через FormData
       if (backgroundImageFile) {