Retry message (re-run last user message with undo)

[Basit-Balogun10]

Description

Add a "Retry" button on the last user message that undoes the AI's response (including file changes) and re-sends the same message to get a different response.

Behavior

• A retry button appears on the most recent user message (or any user message)
• Clicking retry reverts file changes from the AI response and re-submits the message
• Useful when the AI's response was incorrect or took a wrong approach

Related

• Edit user message
• Restore checkpoint

Acceptance Criteria

• Retry button visible on user messages
• File changes from the retried response are reverted
• Message is re-sent to the AI
• Loading state while retrying