* Implement resizable Markdown table of contents panel
Introduce resizable behavior to the Markdown table of contents panel,
allowing users to adjust its width via a draggable handle.
- Persist the custom panel width in the UI state and main process store.
- Clamp the width to a safe range (200px to 600px) based on layout space.
- Ensure the remaining editor workspace does not shrink below 320px
by computing constraints dynamically against the parent container width.
* Clone process env without undefined values for Windows preflight
- Filter out undefined values when cloning process.env to prevent
potential serialization or typing issues.
- Update buildLocalPreflightEnv return type to Record<string, string>.
