feat(ui): add BlockEditor component with block types, slash menu, and drag-and-drop
- add BlockEditor orchestrator with controlled block list and keyboard navigation
- add Block client component with contentEditable sync, drag handles, and markdown shortcuts
- add SlashMenu for inserting block types via `/` command
- add blockRegistry and block type definitions (paragraph, heading, bullet list, numbered list, quote, code, divider)
- add caret and id utility helpers
- export BlockEditor from shared components index
- add BlockEditor demo to admin devkit ComponentsPage
- add README documenting usage and architecture
hykocx
54386d3fe3