Feature: Add cancel button for context condensing operation

[f14XuanLv]

When context condensing is in progress, users currently have no way to cancel the operation. This can be problematic for large contexts that take a long time to process.

We should add a cancel button (X icon) that appears during condensing operations, allowing users to:

• Stop the condensing process immediately
• Continue using the chat without waiting

This would significantly improve the user experience when working with large conversations.