Move SkillEditorProvider from SkillMain to WorkflowAppWrapper so that
store state persists across view switches between Graph and Skill views.
Also add URL query state for view type using nuqs.
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)