Redesign the onboarding shell and create-site form

[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 (the merged wave-1 PRs) so the diff shows only this flow's changes. Once wave 1 merges, this will be retargeted to trunk and marked ready.

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

• The onboarding shell gets the new visual identity: the interactive dot-grid backdrop, window dragging from the edges, view-transition animations between steps, and focus management when sub-pages swap in.
• The create-site form is redesigned to match: Back instead of Cancel, a proper WordPress version selector replacing the free-text field, collapsible advanced settings with inline error counts, and a busy state that shields the form while the site is created.

Testing Instructions

• Start with no sites (or use the Add Site menu) to enter onboarding, open "Create a new site", and exercise the form: version selector, advanced settings validation, and submit. Verify in both light and dark mode.

Pre-merge Checklist

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