A closer match to Ghost's Source theme

Closer match to Ghost Source themes, fewer style clashes, and block editing that works the same in the canvas and sidebar.

• Added smooth scroll support and common Source theme classes for better compatibility with Source-based custom styles.
• Added bordered button color selection for more flexible CTA styling in sections that use bordered button variants.
• Improved nested block editing by supporting context menus consistently in both the preview canvas and sidebar.
• Switched navigation rendering to the standard Ghost {{navigation}} partial for closer Source theme behavior.
• Fixed block field naming and button config migration paths to avoid Ghost rendering conflicts and preserve legacy button content.
• Fixed style conflicts by removing the social links .nav class and correcting bordered color mapping in card-icon and eyebrow blocks.