fix: use same-origin default for auth URL instead of localhost

Avoids ERR_CONNECTION_REFUSED in deployed environments where
VITE_AUTH_URL is not set at build time. Empty-string fallback
routes auth requests to same origin, which the HTTPRoute forwards
to the auth service.

cc @cpfarhood

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/lib/auth-client.ts

@@ -1,7 +1,7 @@
 import { createAuthClient } from "better-auth/react"
 
 export const authClient = createAuthClient({
-  baseURL: import.meta.env.VITE_AUTH_URL ?? "http://localhost:3001",
+  baseURL: import.meta.env.VITE_AUTH_URL || "",
   basePath: "/auth",
 })