- replace `getCaretOffset` with `countCharsUpTo` to treat `<br>` as 1 char
- rewrite `locateOffset` to walk dom tree and account for `<br>` nodes
- add `textLength` helper to compute model-consistent node length
- update `getCaretOffset` and `getCaretRange` to use new counting logic
- 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
hykocx