feat: implement undo button to revert file changes and remove message from history

- Modified handleUndoMessage in server.js to remove the undone message from session history
- Added persistState() call to save the updated session state after undo
- Message is now removed from UI when undo is completed
- Works for opencode/build messages that are completed, errored, or cancelled

chat/public/builder.js

@@ -2153,7 +2153,13 @@ function renderMessages(session) {
   });
 
   // Restore loading indicator visibility after all messages are rendered
-  if (loadingIndicator) {
+  // Only restore if the latest message is not done or error
+  const latestMsg = session.messages?.length > 0 
+    ? [...session.messages].reverse().find(msg => !msg.isBackgroundContinuation)
+    : null;
+  const shouldShowLoading = latestMsg && latestMsg.status !== 'done' && latestMsg.status !== 'error' && latestMsg.status !== 'cancelled';
+  
+  if (loadingIndicator && shouldShowLoading) {
     loadingIndicator.style.visibility = '';
     loadingIndicator.style.position = '';
     // Ensure loading indicator is at the end of chat area
@@ -2163,6 +2169,9 @@ function renderMessages(session) {
       // Move to end to ensure it's after all messages
       el.chatArea.appendChild(loadingIndicator);
     }
+  } else if (loadingIndicator && !shouldShowLoading) {
+    // Remove the loading indicator if message is done/error
+    loadingIndicator.remove();
   }
 
   scrollChatToBottom();
@@ -3040,6 +3049,18 @@ function streamMessage(sessionId, messageId) {
         message.status = 'running';
 
         // Keep loading indicator spinning during streaming - don't hide on first chunk
+        // Update loading indicator text to show we're now building
+        const loadingIndicator = document.getElementById('loading-indicator');
+        if (loadingIndicator) {
+          const statusText = loadingIndicator.querySelector('.loading-status-text');
+          const detailText = loadingIndicator.querySelector('.loading-detail-text');
+          if (statusText && statusText.textContent === 'Starting build process') {
+            statusText.textContent = 'Building plugin';
+          }
+          if (detailText) {
+            detailText.textContent = 'Generating code and files...';
+          }
+        }
 
         // Re-render messages to show new content immediately
         renderMessages(session);