fix: resolve TypeScript errors in entities-treeview

Add explicit property declarations and type annotations to
EntitiesTreeView to fix all 152 "Property X does not exist on type Y"
errors.

- Define EntityTreeViewItem interface extending TreeViewItem with
  entity and _containerUsers properties
- Define EntitiesTreeViewArgs interface extending TreeViewArgs
- Add typed private property declarations for all class members
- Replace ad-hoc intersection types with EntityTreeViewItem
- Add proper casts for protected member access, EventTarget, and
  editor-api runtime properties (apiAsset, apiEntity)
- Fix _updateTreeItemEnabledState to use entity enabled state instead
  of the TreeView element's own enabled state for child recursion

Made-with: Cursor

Author: willeastcott