feat(frontend): set API_BASE_URL and add per-client UUID for polling

- Replace placeholder API_BASE_URL with deployment URL
- Generate per-client ID with crypto.randomUUID() (myClientId)
- Append clientId to GET /messages?since=... requests to support server long-poll tracking

Author: Iswanna

chat-app/frontend/script.js

@@ -1,11 +1,12 @@
-const API_BASE_URL = "${API_BASE_URL}";
+const API_BASE_URL = "https://iswanna-chat-app-backend.hosting.codeyourfuture.io";
+const myClientId = crypto.randomUUID();
 
 let lastIdSeen = -1;
 
 async function getAllMessages() {
   try {
     const response = await fetch(
-      `${API_BASE_URL}/messages?since=${lastIdSeen}`,
+      `${API_BASE_URL}/messages?since=${lastIdSeen}&clientId=${myClientId}`,
     );
 
     const data = await response.json();