Add app-widget embedding to mvmSiteBuilder: a premium app_widget block lets licensed installs pull in public…

pages from other apps via the app-to-app API, backed by a new premium/backend.py that stores and serves each provider's widget list. The text block now runs through a rich-text sanitizer instead of plain escaping. Move mvmSiteBuilder from Creative to Developer Tools and bump it to 1.3.0. QueueDesk 1.2.0 adds a toggle to publish its public page as a site widget for mvmSiteBuilder and a separate toggle to enable/disable its own direct public page.