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/[username]/page.tsx

@@ -84,7 +84,7 @@ export default function UserPage({
 
       // Определяем API URL в зависимости от окружения
       const API = typeof window !== 'undefined' 
-        ? (process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000')  // клиент
+        ? (process.env.NEXT_PUBLIC_API_URL || 'https://links.shareon.kr')  // клиент
         : 'http://web:8000'  // сервер в Docker
 
       console.log('Loading data for user:', usernameValue)

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: {

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) {

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"