Fix auth URLs to use relative paths instead of absolute NEXT_PUBLIC_API_URL

- Change login page to use /api/auth/login/ instead of absolute URL
- Change register page to use /api/auth/register/ instead of absolute URL
- This fixes the 404 error with /auth/undefined/api/auth/login/ path
- Now uses Next.js rewrites from next.config.js correctly

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

@@ -23,7 +23,7 @@ export default function LoginPage() {
     setApiError(null)
     try {
       const res = await fetch(
-        `${process.env.NEXT_PUBLIC_API_URL}/api/auth/login/`,
+        `/api/auth/login/`,
         {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },