feat: Add /nick command usage instruction and broadcast username changes

This commit enhances the chat application by:
- Adding a startup message to inform users about the  command for changing usernames.
- Modifying the  command to broadcast username changes to the chat, using the structured MQTT message format.

src/main.rs

@@ -27,6 +27,7 @@ impl App {
     fn new(username: String) -> App {
         let mut messages = Vec::new();
         messages.push(("System".to_string(), format!("Welcome to the chat, {}!", username)));
+        messages.push(("System".to_string(), "To change your username, type /nick <new_username>".to_string()));
         App {
             messages,
             input: String::new(),
@@ -121,9 +122,20 @@ async fn run_app<B: Backend>(terminal: &mut Terminal<B>, mut app: App) -> io::Re
                         KeyCode::Enter => {
                             let message_text = app.input.drain(..).collect::<String>();
                             if message_text.starts_with("/nick ") {
+                                let old_username = app.username.clone();
                                 let new_username = message_text.split_whitespace().nth(1).unwrap_or(&app.username).to_string();
-                                app.username = new_username;
+                                app.username = new_username.clone();
                                 app.messages.push(("System".to_string(), format!("Username changed to: {}", app.username)));
+
+                                let chat_message = ChatMessage {
+                                    username: old_username,
+                                    message: format!("has changed their name to {}", new_username),
+                                };
+                                let payload = serde_json::to_string(&chat_message).unwrap();
+                                client_clone
+                                    .publish("chat/msg", QoS::AtMostOnce, false, payload.as_bytes())
+                                    .await
+                                    .unwrap();
                             } else {
                                 let chat_message = ChatMessage {
                                     username: app.username.clone(),