- Add constants.ts with ROOT_ID, CONTEXT_MENU_TYPE, NODE_MENU_TYPE
- Add root utilities to tree-utils.ts (isRootId, toApiParentId, etc.)
- Replace '__root__' with ROOT_ID for consistent root identifier
- Replace inline 'blank'/'root' strings with constants
- Use NodeMenuType for type-safe menu type props
- Remove duplicate ContextMenuType from types.ts, use from constants.ts