feat(BlockEditor): add inline formatting with rich content model

- migrate block content from plain strings to InlineNode[] structure
- add inline toolbar (bold, italic, code, color, link) on text selection
- add checklist block type with toggle support
- add image block type (URL-based, phase 2)
- add inline serialization helpers (inlineToDom, domToInline)
- add inline types and length utilities
- extend caret utils with range get/set support
- update block registry and all existing block types for new content model
- update demo blocks in ComponentsPage to use rich inline content
- update README to reflect new architecture

src/shared/components/BlockEditor/SlashMenu.client.js

@@ -16,9 +16,11 @@ const SHORTCUT_HINT = {
   heading_6: '######',
   bullet_item: '-',
   numbered_item: '1.',
+  checklist: '[]',
   quote: '>',
   code: '```',
   divider: '---',
+  image: '',
 };
 import { listBlocks } from './blockRegistry.js';