fix(mcp): derive OAuth login redirect and well-known discovery from request base URL

apps/nextjs/src/app/api/mcp/oauth/authorize/route.ts

@@ -1,5 +1,6 @@
 import type { NextRequest } from "next/server";
 
+import { extractBaseUrlFromHeaders } from "@homarr/common";
 import { auth } from "@homarr/auth/next";
 
 import { consumePendingAuth, createAuthCode, getClient, storePendingAuth } from "../_store";
@@ -137,7 +138,8 @@ export async function GET(req: NextRequest) {
     );
   }
 
-  const loginUrl = new URL("/auth/login", url.origin);
+  const baseUrl = extractBaseUrlFromHeaders(req.headers);
+  const loginUrl = new URL("/auth/login", baseUrl);
   loginUrl.searchParams.set("callbackUrl", `/api/mcp/oauth/authorize?pending=${id}`);
   return Response.redirect(loginUrl.toString());
 }