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

src/shared/components/index.js

@@ -21,6 +21,7 @@ export { default as StatCard } from './StatCard';
 export { default as Table } from './Table';
 export { default as Textarea } from './Textarea';
 export { default as MarkdownEditor } from './MarkdownEditor';
+export { default as BlockEditor } from './BlockEditor';
 export { default as PasswordStrengthIndicator } from './PasswordStrengthIndicator';
 export { default as FilterTabs } from './FilterTabs';
 export { default as Breadcrumb } from './Breadcrumb';