Use FormBuilder to build backend form dropdown

[mjauvin]

Depends on wintercms/storm#222
Related to wintercms/docs#253

This also adds support for optGroup in dropdown through the FormBuilder.

Summary by CodeRabbit

• Refactor
  • Consolidated dropdown rendering into a single, consistent path for improved reliability and easier maintenance.
  • Preserves user-facing behaviors: placeholder, optional search, custom-option entry, empty-option handling, and preview/read-only display.
  • Error and accessibility handling unified to reduce rendering inconsistencies.

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[LukeTowers]

@coderabbitai review this