bug(desktop): changing thinking level moves the composer caret to the start

[Sun-GLiang]

What happened

Changing the model thinking level while text is being edited in the conversation composer moves the caret to the beginning of the draft.

Expected behavior: changing the thinking level should preserve both the draft text and the current caret/selection position, so the user can continue typing from the same location.

How to reproduce

1. Open a conversation in the Desktop app.
2. Enter some text in the conversation composer.
3. Place the caret somewhere other than the beginning of the draft.
4. Change the model thinking level.
5. Observe that the caret moves to the beginning of the draft instead of remaining at its previous position.

Environment

• Maka version or commit: 0.2.0 / 7fdc015da
• OS and version: macOS 15.6 (arm64)
• Surface: Desktop
• Node.js version, if running from source: v24.19.0

Logs, screenshots, or additional context

This interrupts continuous typing and can cause subsequent text to be inserted at the wrong location, especially when editing a longer draft.

[seekskyworld]

/take

我来处理这个 Desktop composer caret 问题。会先补充切换 thinking level 时保留草稿 selection 的回归测试，再提交最小修复。