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/server.js

@@ -15920,6 +15920,16 @@ async function handleUndoMessage(req, res, sessionId, messageId, userId) {
       log('Cleared todos from undone message', { sessionId, messageId, clearedCount: todoCount });
     }
 
+    // Remove the message from session history
+    const messageIndex = session.messages.findIndex(m => m.id === messageId);
+    if (messageIndex !== -1) {
+      session.messages.splice(messageIndex, 1);
+      log('Removed message from history', { sessionId, messageId, remainingMessages: session.messages.length });
+    }
+
+    // Persist the updated state
+    await persistState();
+
     log('Undo command completed', { sessionId, messageId });
     sendJson(res, 200, { ok: true, message: 'Undo command sent successfully' });
   } catch (error) {