Context

When typing on the read-only queued message editor, we push the content to the main, editable message editor. However, the cursor position was being reset when that happened. This PR fixes that by tracking the cursor offset when doing that, also accounting for the case where there could be pre-existing content in the message editor.

How to Review

• Queue a message in the agent panel
• Place your cursor somewhere in the middle of your queued message and type something
• See how the cursor position is preserved once it goes down to the main message editor

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Agent: Fix cursor position being reset when editing queued messages.