feat: introduce backend url environment variable

ref: N25B-352

src/pages/ConnectedRobots/ConnectedRobots.tsx

@@ -2,6 +2,7 @@
 // University within the Software Project course.
 // © Copyright Utrecht University (Department of Information and Computing Sciences)
 import { useEffect, useState } from 'react'
+import { API_BASE_URL } from '../../config/api.ts';
 
 /**
  * Displays the current connection status of a robot in real time.
@@ -25,7 +26,7 @@ export default function ConnectedRobots() {
   useEffect(() => {
     // Open a Server-Sent Events (SSE) connection to receive live ping updates.
     // We're expecting a stream of data like that looks like this: `data = False` or `data = True`
-    const eventSource = new EventSource("http://localhost:8000/robot/ping_stream");
+    const eventSource = new EventSource(`${API_BASE_URL}/robot/ping_stream`);
     eventSource.onmessage = (event) => {
     
       // Expecting messages in JSON format: `true` or `false`