The visibility of preview panel is controlled by `showPreviewPanel` in
`ChatPanel`, but there is no entrance to open it in this PR.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
- **New Features**
- Introduced a split-view layout in the chat panel, allowing users to
view both the chat and a new preview panel side by side.
- Added a draggable divider for resizing the chat and preview panels,
with the divider position saved automatically for future sessions.
- **Refactor**
- Updated the chat panel interface to support the new split-view and
preview panel functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->