feat: introduce backend url environment variable

ref: N25B-352

test/pages/monitoringPage/MonitoringPageAPI.test.ts

@@ -10,6 +10,7 @@ import {
     useExperimentLogger, 
     useStatusLogger 
 } from '../../../src/pages/MonitoringPage/MonitoringPageAPI';
+import { API_BASE_URL } from '../../../src/config/api.ts';
 
 // --- MOCK EVENT SOURCE SETUP ---
 // This mocks the browser's EventSource so we can manually 'push' messages to our hooks
@@ -72,7 +73,7 @@ describe('MonitoringPageAPI', () => {
       await sendAPICall('test_type', 'test_ctx');
 
       expect(globalThis.fetch).toHaveBeenCalledWith(
-        'http://localhost:8000/button_pressed',
+        `${API_BASE_URL}/button_pressed`,
         expect.objectContaining({
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
@@ -220,4 +221,4 @@ describe('MonitoringPageAPI', () => {
       expect(consoleSpy).toHaveBeenCalledWith('Status stream error:', expect.any(Error));
     });
   });
-});
+});