Support AuthUI preview mode

[linear[bot]]

• Support query param
  • maybe x_preview=inline
• Page to be supported
  • [X] Login
  • [ ] Signup
  • [ ] Verification code
  • [ ] Create Password
  • [ ] Enter Password
  • [ ] Passkey
  • [ ] 2-step verification
  • [ ] log in link
• Checking if in preview mode, Check Preview to render html for preview mode
  • https://github.com/authgear/authgear-server/blob/90da43176deebd4fd3070c8585e1f04ded4a4e06/pkg/auth/handler/webapp/authflowv2/login.go#L199

[linear[bot]]

DEV-1424 Support AuthUI preview mode