As part of the ongoing migration away from styled-components, this PR removes all styled-components usage from the TreeViewWithLeadingAction.stories.tsx file and replaces it with CSS Modules.

Changes Made

Removed styled-components dependencies:

• Removed Box component imports and usage
• Replaced all sx prop usage with CSS classes
• Created a new CSS Module file for story-specific styling

Migration details:

• <Box sx={{maxWidth: 400}}> → <div className={classes.StoryContainer}>
• <Box> wrapper → <div>
• sx={{color: 'done.fg'}} → className={classes.DoneIcon} using --fgColor-done
• sx={{color: 'open.fg'}} → className={classes.OpenIcon} using --fgColor-open

New CSS Module file: TreeViewWithLeadingAction.stories.module.css

.StoryContainer {
  max-width: 400px;
}

.DoneIcon {
  color: var(--fgColor-done);
}

.OpenIcon {
  color: var(--fgColor-open);
}

The story functionality remains exactly the same - this is a pure refactoring from styled-components to CSS Modules with no visual or behavioral changes.

Fixes #6528.

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