- Add undoStack/redoStack refs with pushHistory/handleUndo/handleRedo
- Replace execCmd('undo')/execCmd('redo') with custom stack handlers
- Call pushHistory before structural changes (delete, insert field/table/image, formatting)
- Add onMouseDown preventDefault to toolbar and field library buttons to stop focus loss
- Implement saveSelection/restoreSelection using savedRangeRef
- Bind onBlur/onMouseUp/onKeyUp on editor to persist cursor position
- Restore selection in insertSmartField and insertImage before insertHTML
- Update experience record (#20)
