fix: frontend API_BASE_URL respects empty VITE_API_URL for same-origin deploy

Amp-Thread-ID: https://ampcode.com/threads/T-019c22db-46b4-7732-8f60-50293667624e
Co-authored-by: Amp <amp@ampcode.com>

frontend/src/config/api.ts

@@ -1,11 +1,20 @@
 /**
  * Single source for API and WebSocket base URLs.
  * Uses VITE_API_URL so one env var works for both REST and WebSocket.
+ *
+ * When VITE_API_URL is defined (even as empty string), use it as-is.
+ * Empty string = same origin (for HF Spaces / single-container deploy).
+ * Only fall back to localhost:8000 when the var is completely undefined.
  */
-const raw = typeof import.meta.env.VITE_API_URL === 'string' ? import.meta.env.VITE_API_URL.trim() : '';
-const API_BASE_URL = raw ? raw.replace(/\/+$/, '') : 'http://localhost:8000';
+const envVal = import.meta.env.VITE_API_URL;
+const API_BASE_URL =
+  typeof envVal === 'string'
+    ? envVal.trim().replace(/\/+$/, '')
+    : 'http://localhost:8000';
 
 /** WebSocket base URL derived from API URL (http -> ws, same host/port) */
-const WS_BASE_URL = API_BASE_URL.replace(/^http/, 'ws');
+const WS_BASE_URL = API_BASE_URL
+  ? API_BASE_URL.replace(/^http/, 'ws')
+  : `${window.location.protocol === 'https:' ? 'wss:' : 'ws:'}//${window.location.host}`;
 
 export { API_BASE_URL, WS_BASE_URL };