fix: use relative URLs in web mode to leverage Vite proxy

In web mode, the API client was hardcoding localhost:3008, which bypassed
the Vite proxy and caused CORS errors. Now it uses relative URLs (just /api)
in web mode, allowing the proxy to handle routing and making requests appear
same-origin.

- Web mode: Use relative URLs for proxy routing (no CORS issues)
- Electron mode: Continue using hardcoded localhost:3008

This allows the Vite proxy configuration to actually work in web mode.

Fixes: Persistent CORS errors in web mode development

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

apps/ui/src/lib/http-api-client.ts

@@ -156,6 +156,12 @@ const getServerUrl = (): string => {
   if (typeof window !== 'undefined') {
     const envUrl = import.meta.env.VITE_SERVER_URL;
     if (envUrl) return envUrl;
+
+    // In web mode (not Electron), use relative URL to leverage Vite proxy
+    // This avoids CORS issues since requests appear same-origin
+    if (!window.electron) {
+      return '';
+    }
   }
   // Use VITE_HOSTNAME if set, otherwise default to localhost
   const hostname = import.meta.env.VITE_HOSTNAME || 'localhost';