fix(frontend): use configured API base for direct requests

Route direct fetch calls, setup requests, gateway usage checks, and ops WebSocket connections through the configured API base instead of assuming same-origin backend paths.

Also derive OAuth callback suggestions from the configured API base so split frontend/API deployments show usable redirect URLs.

frontend/src/api/setup.ts

@@ -2,10 +2,11 @@
  * Setup API endpoints
  */
 import axios from 'axios'
+import { buildGatewayUrl } from './url'
 
 // Create a separate client for setup endpoints (not under /api/v1)
 const setupClient = axios.create({
-  baseURL: '',
+  baseURL: buildGatewayUrl('/').replace(/\/+$/, ''),
   timeout: 30000,
   headers: {
     'Content-Type': 'application/json'