[Migration] Migrate Form Components from Sass to PostCSS

[seanwuapps]

Objective

Migrate all form component styles from Sass to PostCSS.

Components to Migrate

• go-checkbox
• go-datepicker
• go-field
• go-fieldset
• go-radio
• go-select
• go-switch
• go-textarea

For each component:

1. Create a new .css file
2. Migrate Sass features to PostCSS equivalents
3. Update component's styleUrl property
4. Update tests if needed
5. Verify visual consistency
6. Test across supported browsers

Dependencies

• Depends on [Migration] Replace Sass with PostCSS: Global Styles and Utils #278 for global styles migration
• Update component documentation after migration

Migration Guidelines

• Use CSS custom properties for variables
• Use PostCSS nesting for nested selectors
• Replace Sass functions with CSS calculations where possible
• Document any breaking changes