Add the Connect a site onboarding flow

[shaunandrews]

Related issues

• Part of the site-creation onboarding redesign — see the umbrella PR (linked below once open) for the full plan.
• Draft note: based on site-creation-foundation; will be retargeted to trunk and marked ready once wave 1 merges.

How AI was used in this PR

Split out of the workbench/site-creation exploration branch with Claude Code; designed and tested by Shaun on the original branch.

Proposed Changes

• A new onboarding flow for starting from an existing WordPress.com or Pressable site: sign in, pick from a grouped site list (with environment badges, screenshots, search above five sites, and already-connected sites marked), and Studio creates a local copy and pulls the remote site into it.
• Handles the unhappy paths: offline state, accounts with no eligible sites, and cleanup of the local site if the pull fails mid-flight.
• The route is registered but not yet linked from onboarding home — that lands with the home redesign, so this PR is inert until then.

Testing Instructions

• Navigate to /onboarding/connect directly (or wait for the home redesign PR), sign in to WordPress.com, and connect a site. Verify environment badges, search behavior with larger accounts, keyboard navigation, and both color schemes.

Pre-merge Checklist

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