Agentic UI: Combine create and blueprint onboarding into Build a new site

[shaunandrews]

Related issues

• Related to Agentic UI: Site-creation onboarding redesign — umbrella (do not merge) #3797

How AI was used in this PR

AI helped split the larger site-creation experiment into a smaller behavior-focused change. I reviewed the resulting diff and ran the checks listed below.

Proposed Changes

• Combines the separate “Create new” and “Start from a blueprint” onboarding choices into one “Build a new site” entry point.
• Moves the blank-site choice into the blueprint/start-point selector as an “Empty site” option, so users pick the kind of new site after choosing to build.
• Keeps the existing create and blueprint flows intact; this only changes how users enter them.

Testing Instructions

• Open the app UI onboarding home.
• Confirm the home screen offers “Build a new site” instead of separate create/blueprint cards.
• Choose “Build a new site” and confirm the selector includes “Empty site” plus featured blueprints.
• Choose “Empty site” and confirm it opens the normal create-site form.

Validation run:

• npx eslint --fix apps/ui/src/components/blueprint-selector/index.tsx apps/ui/src/ui-classic/router/route-onboarding-blueprint/index.tsx apps/ui/src/ui-classic/router/route-onboarding-create/index.tsx apps/ui/src/ui-classic/router/route-onboarding-home/index.tsx apps/ui/src/ui-desks/onboarding/index.tsx
• npm run typecheck
• npm test -- apps/studio/src/modules/add-site/tests/add-site.test.tsx apps/studio/src/modules/add-site/hooks/tests/use-blueprint-deeplink.test.tsx

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?