feat: introduce backend url environment variable

ref: N25B-352

test/pages/robot/Robot.test.tsx

@@ -3,6 +3,7 @@
 // © Copyright Utrecht University (Department of Information and Computing Sciences)
 import { render, screen, act, cleanup, fireEvent } from '@testing-library/react';
 import Robot from '../../../src/pages/Robot/Robot';
+import { API_BASE_URL } from '../../../src/config/api.ts';
 
 // Mock EventSource
 const mockInstances: MockEventSource[] = [];
@@ -64,7 +65,7 @@ describe('Robot', () => {
     await act(async () => fireEvent.click(button));
 
     expect(globalThis.fetch).toHaveBeenCalledWith(
-      'http://localhost:8000/message',
+      `${API_BASE_URL}/message`,
       expect.objectContaining({
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
@@ -83,7 +84,7 @@ describe('Robot', () => {
     );
 
     expect(globalThis.fetch).toHaveBeenCalledWith(
-      'http://localhost:8000/message',
+      `${API_BASE_URL}/message`,
       expect.objectContaining({
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },