A faster builder and more reliable theme downloads

The builder should feel faster day to day. Theme downloads and project upgrades fail less often in practice.

• Added rich text field support and safer reserved field name handling.
• Added automatic migration for outdated block and section settings when you open older projects.
• Improved preview performance with less stutter while you edit.
• Improved loading states, auth flow reliability, and project switching UX.
• Fixed theme packaging to better detect used templates/styles and avoid broken download cases.