fix: use forwarded host for auth redirects

lib/request-url.ts

@@ -0,0 +1,19 @@
+import { NextRequest } from "next/server";
+
+export function getRequestBaseUrl(request: NextRequest) {
+  const configured = process.env.APP_URL?.trim();
+  if (configured) {
+    return new URL(configured);
+  }
+
+  const forwardedHost = request.headers.get("x-forwarded-host");
+  const forwardedProto = request.headers.get("x-forwarded-proto");
+  const host = forwardedHost?.split(",")[0]?.trim() || request.headers.get("host") || request.nextUrl.host;
+  const proto = (forwardedProto?.split(",")[0]?.trim() || request.nextUrl.protocol || "http").replace(":", "");
+
+  if (!host) {
+    return request.nextUrl;
+  }
+
+  return new URL(`${proto}://${host}`);
+}