use SITE_URL via helper instead of custom env var

pages/api/auth/[...all].ts

@@ -1,3 +1,4 @@
+import { getBaseURL } from "../../../helpers/url"
 import { auth } from "../../../lib/auth"
 import type { NextApiRequest, NextApiResponse } from "next"
 
@@ -8,7 +9,7 @@ export const config = {
 }
 
 export default async function handler(req: NextApiRequest, res: NextApiResponse) {
-  const baseUrl = process.env.BETTER_AUTH_URL || "http://localhost:3000"
+  const baseUrl = getBaseURL()
   const url = new URL(req.url!, baseUrl)
 
   let body: Buffer | undefined
@@ -34,4 +35,4 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
     res.setHeader(key, value)
   })
   res.end(await response.text())
-}
+}