Agentic UI: Polish the Build a new site gallery and create form

[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 isolate the gallery/form polish from the larger proof-of-concept branch. I reviewed the split and ran lint, typecheck, and focused tests.

Proposed Changes

• Makes the Build a new site selector feel more like a first-class chooser, with a clearer empty-site card, curated featured blueprints, preview affordances, and upload access.
• Improves the create-site form with a fixed action footer, busy overlay, and a WordPress version selector instead of free text.
• Keeps the change focused on the “build a new site” path rather than adding connect/import/desks behavior.

Testing Instructions

• Open the Build a new site/blueprint selector.
• Confirm “Empty site” appears as a normal starting point and featured blueprints are presented as selectable cards.
• Preview a blueprint and upload a valid/invalid blueprint file.
• Open the create-site form and confirm the footer actions, loading state, and WordPress version picker behave correctly.

Validation run:

• npx eslint --fix apps/studio/src/constants.ts apps/studio/src/modules/add-site/components/new-site-options.tsx apps/ui/src/components/blueprint-selector/index.tsx apps/ui/src/components/create-site-form/index.tsx apps/ui/src/components/site-fields/index.ts apps/ui/src/components/busy-overlay/index.tsx apps/ui/src/components/onboarding-footer/index.tsx apps/ui/src/data/core/connectors/ipc/index.ts apps/ui/src/data/core/types.ts apps/ui/src/data/queries/use-wordpress-versions.ts apps/ui/src/hooks/use-grid-arrow-navigation.ts apps/ui/src/ui-classic/router/route-onboarding-blueprint/index.tsx apps/ui/src/ui-desks/onboarding/index.tsx tools/common/constants.ts tools/common/lib/blueprint-curation.ts
• 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?