Implement MVP features for skill editor based on design doc:
- Add Zustand store with Tab, FileTree, and Dirty slices
- Rewrite file tree using react-arborist for virtual scrolling
- Implement Tab↔FileTree sync with auto-reveal on tab activation
- Add upload functionality (new folder, upload file)
- Implement Monaco editor with dirty state tracking and Ctrl+S save
- Add i18n translations (en-US and zh-Hans)