Email Editor adds new sections and smarter styling

The email editor now includes headings (H1–H3), dividers, spacers, and 2–3 column layouts alongside improved organization. Sections are grouped into categories (Basic Elements, Lists, Layouts) for easier discovery, and all are fully editable after insertion. You can also use horizontal and vertical layout controls to position items precisely. New sections automatically adopt your template's existing styling, so you maintain visual consistency whether you're building manually or using AI-generated content.