Agentic UI: Move backup import picking to the onboarding home card

[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 extract this import-flow behavior from the larger branch. I reviewed the split and ran the checks listed below.

Proposed Changes

• Moves backup file selection to the onboarding home card so users pick or drop the backup before entering the import form.
• Makes the import route focus on configuring the new local site after a backup has already been selected.
• Redirects direct/refresh visits without a pending backup back to the onboarding home picker.

Testing Instructions

• Open onboarding and use the import card to browse for a supported backup file.
• Drop a supported backup file onto the import card.
• Confirm unsupported files show an inline error.
• Confirm a valid file opens the configure imported site form with a filename-derived site name.
• Refresh or directly open /onboarding/import and confirm it returns to the onboarding home.

Validation run:

• npx eslint --fix apps/ui/src/lib/backup-files.ts apps/ui/src/lib/pending-slot.ts apps/ui/src/lib/pending-backup.ts apps/ui/src/ui-classic/router/route-onboarding-home/index.tsx apps/ui/src/ui-classic/router/route-onboarding-import/index.tsx
• npm run typecheck
• npm test -- apps/studio/src/modules/add-site/tests/add-site.test.tsx

Pre-merge Checklist

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