feat(admin): reuse user ui in branding previews

[markwylde]

Summary

• expose shared user-ui surfaces so branding previews can render the same components as the live user pages
• replace bespoke admin Apps, Security, Authorize, and Profile previews with shared user-ui components
• add admin and Playwright coverage for shared branding preview behavior

Verification

• pnpm tidy
• pnpm build
• pnpm test