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/BlockEditor/index.js

@@ -0,0 +1,21 @@
+// Barrel public du BlockEditor.
+//
+// Importer le composant depuis :
+//   import { BlockEditor } from '@zen/core/shared/components';
+//
+// Pour enregistrer un type de bloc custom dans une app consommatrice :
+//   import { registerBlock } from '@zen/core/shared/components/BlockEditor';
+//   registerBlock({ type: 'kpi', label: 'KPI', icon: '📊', isText: false,
+//                   create: () => ({ id: crypto.randomUUID(), type: 'kpi' }),
+//                   Component: MyKpiBlock });
+
+export { default as BlockEditor } from './BlockEditor.client.js';
+export { default } from './BlockEditor.client.js';
+export {
+  registerBlock,
+  getBlockDef,
+  listBlocks,
+  isBlockText,
+  DEFAULT_BLOCK_TYPE,
+} from './blockRegistry.js';
+export { newBlockId } from './utils/ids.js';