Agentic UI: Polish onboarding accessibility and visual behavior

[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 accessibility and motion polish out of the broader branch. I reviewed the changes and ran validation.

Proposed Changes

• Announces successful site creation/import to assistive technology before navigating away.
• Moves keyboard focus to the incoming onboarding page heading after route/step changes unless the page has already focused a control.
• Enables lightweight route view transitions for onboarding while respecting reduced motion.
• Fixes the non-keyboard focus styling so custom onboarding buttons do not flash the wrong background on click.

Testing Instructions

• Navigate between onboarding home, create, blueprint select/configure, and import steps using the keyboard.
• Confirm focus follows the new page heading after navigation.
• Complete each creation path and confirm screen-reader live announcements fire.
• Confirm route transitions are disabled with reduced-motion enabled.

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-import/index.tsx apps/ui/src/ui-classic/router/router.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?