Allow panels to open in floating windows

### Feature summary

Allow features like the editor, diff viewer, and terminal to be opened in separate floating windows.

### What problem are you trying to solve?

The desktop app now includes many useful tools, which is great because everything can stay in one place.

But on a multi-monitor setup, all these tools are still inside the same main window. I would like to keep Copilot on one screen and move the editor, diff, or terminal to another screen.

### Proposed solution

Allow panels such as the editor, diff viewer, and terminal to be detached from the main window and opened as independent windows.

Similar to how Photoshop lets you undock panels and move them to other screens.

### Workflow impact

This would make multi-monitor setups much better and reduce the need to switch between panels.

For example:

- Main screen: Copilot
- Second screen: editor or diff
- Third screen: terminal

### Installation context

_No response_

### Additional context

_No response_

该 issue 没有指明任何文件、测试或入口点。首先定位桌面应用如何创建编辑器、差异查看器和终端面板，以及窗口如何进行管理；当这些面板可以分离为独立窗口并在多个显示器之间移动时，即视为完成。

由索引模型根据 Issue 内容生成。