refactor: export joinUrl from auth/api and reuse in lib/api

Single source of truth for the VITE_API_BASE_URL prefix logic. Both
auth/api and lib/api imported the same BASE_URL const; promote joinUrl
to a shared exported helper so future endpoints don't drift on the
trailing-slash normalization.

src/lib/api.ts

@@ -47,6 +47,7 @@ import type {
 import {
   authedFetch,
   authedFetchResponse,
+  joinUrl,
 } from "@/auth/api";
 
 const BASE_URL = (import.meta.env.VITE_API_BASE_URL as string | undefined) ?? "";