Agentic UI: Add Connect a site to onboarding

[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 fold review feedback about large WordPress.com accounts into the Connect-site slice. I reviewed the code and ran lint, typecheck, and focused tests.

Proposed Changes

• Adds a Connect a site onboarding route for bringing WordPress.com or Pressable sites into the app UI.
• Loads syncable sites page-by-page during the connect step, with search and “load more” behavior, instead of fetching an entire account up front.
• Shows already-connected and unsupported sites distinctly while only allowing syncable sites to be selected.
• Creates the local shell without starting it, connects it to the remote site, starts the pull, and rolls back the shell if connect setup fails.

Testing Instructions

• Open onboarding and choose “Connect a site.”
• Test signed-out, offline, empty-account, single-site, and multi-site states if available.
• On an account with many sites, confirm the first page loads without fetching every site and that “Load more”/scroll loading fetches additional pages.
• Select a syncable site and confirm the flow creates the local site, connects it, starts pull, and navigates to the new site view.

Validation run:

• npx eslint --fix apps/studio/src/ipc-handlers.ts apps/studio/src/modules/sync/lib/ipc-handlers.ts apps/studio/src/preload.ts apps/ui/src/components/busy-overlay/index.tsx apps/ui/src/components/onboarding-footer/index.tsx apps/ui/src/components/onboarding-layout/index.tsx apps/ui/src/data/core/connectors/ipc/index.ts apps/ui/src/data/core/index.ts apps/ui/src/data/core/types.ts apps/ui/src/data/queries/use-create-site-helpers.ts apps/ui/src/data/queries/use-wpcom-sites.ts apps/ui/src/data/queries/use-wpcom-sites.test.tsx apps/ui/src/hooks/use-grid-arrow-navigation.ts apps/ui/src/hooks/use-offline.ts apps/ui/src/lib/docs-links.ts apps/ui/src/ui-classic/router/layout-onboarding/index.tsx apps/ui/src/ui-classic/router/route-onboarding-connect/index.tsx apps/ui/src/ui-classic/router/route-onboarding-home/index.tsx apps/ui/src/ui-classic/router/router.tsx tools/common/lib/sync/environment-utils.ts tools/common/lib/sync/mshots.ts tools/common/lib/sync/sync-api.ts tools/common/lib/sync/transform-sites.ts
• npm run typecheck
• npm test -- apps/ui/src/data/queries/use-wpcom-sites.test.tsx apps/studio/src/modules/add-site/tests/add-site.test.tsx apps/studio/src/modules/add-site/hooks/tests/use-find-available-site-name.test.ts 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?