Update Box.stories.tsx to no longer use styled-components

[Copilot]

This PR removes styled-components usage from the Box component Storybook stories as part of the ongoing migration away from the library.

Changes Made

Created CSS Module for Story Styles

• Added Box.stories.module.css with equivalent styles for the previous sx prop usage
• Converted styled-system values to CSS design tokens:
  • borderWidth: 1 → border-width: var(--borderWidth-thin)
  • borderStyle: 'solid' → border-style: solid
  • borderColor: 'border.default' → border-color: var(--borderColor-default)
  • p: 3 → padding: var(--base-size-12)

Updated Story Components

• Default story: Replaced <Box>Default Box</Box> with <div>Default Box</div>
• Playground story: Replaced Box usage with dynamic HTML element creation based on the as prop, applying CSS module classes instead of the sx prop
• Added imports for clsx utility and CSS module classes
• Removed the sx prop from Playground.args while preserving the as prop functionality

Maintained Compatibility

• Kept the Box import for component meta definition (required for Storybook)
• Preserved all existing argTypes configuration
• Maintained the same visual appearance and interactive functionality

The stories now render identically to before but use CSS modules instead of styled-components, reducing the codebase's dependency on the deprecated styling approach.

Fixes #6577.

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