Added ability to toggle header cells directly from the table cell menu. This enhancement includes:
- New toggle header cell button with IconTableRow icon
- Consistent UI/UX with existing table menu patterns
- Proper internationalization support

- Extended TableCell and TableHeader to support backgroundColor attribute
- Created TableBackgroundColor component with 21 color options
- Integrated color picker into table cell menu using Mantine UI
- Added support for both regular cells and header cells
- Updated imports to use custom TableHeader from @docmost/editor-ext

- Created TableTextAlignment component with left, center, and right alignment options
- Integrated alignment selector into table cell menu
- Shows current alignment icon in the button
- Displays checkmark next to active alignment in dropdown