Related issues

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

How AI was used in this PR

AI helped isolate the duplicate-code removal from the broader onboarding experiment. I reviewed the final diff and ran validation.

Proposed Changes

• Reuses the ui-classic onboarding pages inside Desks instead of maintaining a separate copy.
• Adds small route adapters so shared onboarding exit URLs land on the correct Desks destinations.
• Keeps existing onboarding behavior the same while reducing duplicate flow code.

Testing Instructions

• Open Desks onboarding and confirm the home, create, blueprint, and import flows still render.
• Create or import a site from Desks onboarding and confirm the final navigation lands on the Desks site route.
• Close onboarding with existing sites present and confirm it returns to the Desks root.

Validation run:

• npx eslint --fix apps/ui/src/ui-classic/router/layout-onboarding/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-classic/router/route-onboarding-import/index.tsx apps/ui/src/ui-desks/app.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?