- Update variables.css surfaces to match brand navy (#152132)
- Add sidebar-bg and input-bg CSS variables with fallbacks
- Add "Reset to Brand Identity" button in branding settings
- Add resetColors controller action that reverts all colors to
  their default_value from platform_theme
- Reset live DB colors to brand defaults
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>