🎨 Palette: Improve checkbox accessibility

[vizzyfreezy]

Improved the accessibility of the Checkbox component.

Changes:

• Replaced display: none on the hidden input element with standard "visually hidden" styles (opacity: 0, position: absolute, clip: rect(...)). This ensures the input remains in the accessibility tree and is focusable via keyboard.
• Added :focus-visible styles to the adjacent .box element to provide a clear focus indicator (using box-shadow) when the checkbox is focused via keyboard.

Verification:

• Verified visually using a mocked HTML page and Playwright script (verification/verify_checkbox.py), confirming that the focus ring appears correctly when the checkbox is focused.
• Verified that the component structure in src/components/checkbox/index.js supports the CSS sibling selector (input + .box).
• Ran pnpm check to ensure no linting errors.

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PR created automatically by Jules for task 16963116899284669631 started by @vizzyfreezy

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