Add PreviewStep page (wizard step 4)

Informational review step — no form fields. List-detail split:
ProviderCardCompact + "What happens next" numbered checklist + CTAs
(left), PackageDetail breakdown (right). Pre-planning variant shows
"Explore other options" tertiary CTA. Checklist reduces anxiety about
remaining steps. Pure presentation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/pages/PreviewStep/index.ts

@@ -0,0 +1,2 @@
+export { default } from './PreviewStep';
+export * from './PreviewStep';