feat: Checkbox 2.0 component - #4208

- Checkbox now uses Field component for layout and accessibility
- Simplified API: removed labelProps and wrapperProps
- Added indicator prop for required/optional text
- Updated CSS with Figma design values (gap, padding, focus frame)
- Added Compact story to demonstrate density support
- Fixed Storybook preview to import correct CSS file
- Moved CSS entry point to next/index.css

- Checkbox now supports helperMessage for validation feedback
- Helper message automatically connected via aria-describedby
- Error state sets role='alert' on helper message for a11y
- Helper message color changes to red when error=true
- Added WithHelperMessage and updated ErrorState stories
- Added tests for helperMessage functionality

- Removed error prop as it's not in the Figma design
- Users can use data-color-appearance attribute directly if needed
- Kept helperMessage for additional context
- Removed ErrorState story and related test

Shows how to use data-color-appearance='danger' for error styling

- Default to 'accent' color appearance for checkbox icon
- Allow override via data-color-appearance prop (e.g., 'danger')
- Helper message stays neutral by default, turns red for danger
- Add ColorAppearance type with valid values
- Add tests for color appearance functionality

… CSS

Token system handles data-color-appearance automatically via --eds-color-text-subtle

- Replace hardcoded pixel values with EDS tokens and future Figma token fallbacks
- Simplify CSS selectors by removing duplicates
- Move ColorAppearance type to shared types.ts
- Add data-density support for Table checkbox story
- Simplify helper message color logic (neutral for accent, follows color-appearance otherwise)

Replaced padding-block + min-height with fixed height from density token.
This ensures comfortable density gets correct 24px height (was 32px due
to hardcoded 10px padding fallback designed for spacious mode).

Flexbox centers content vertically, so no padding needed.

- Remove WithIndicator story (indicator prop still supported via Field)
- Remove inline error message from form example (not relevant for checkbox)
- Remove data-density from TableCheckbox (uses default spacious)

Changed from opacity: 0.4 on teal to --eds-color-border-neutral-medium
(#aeaeae) to match Figma design exactly.

- Add comfortable density padding for standalone checkbox (28px)
- Spacious remains 36px (24px icon + 12px padding)
- Comfortable is 28px (20px icon + 8px padding)
- Clean up stories with dedicated WithoutVisibleLabel story
- Fix ColorSchemes story layout
- Update index.css layer order to match input branch

- Add argTypes with categorized props (Core, States, Field, Styling, HTML)
- Move className to input element via ...rest for consistency
- Remove data-color-appearance from public argTypes
- Update test to reflect className on input element

- Add Figma Code Connect file for design-code mapping
- Organize tests into descriptive describe blocks
- Fix import order in stories per conventions
- Remove redundant storyName annotations
- Enable and add snapshot test

Native disabled attribute already communicates disabled state to assistive technologies, making aria-disabled redundant on form controls.

…x test