fix: reduce CodeBlock button z-index to prevent overlap with popovers

[daniel-lxs]

Description

This PR fixes an issue where the CodeBlock language dropdown and copy button were appearing above popovers and other overlay elements.

Problem

The CodeBlockButtonWrapper component had a z-index: 100, which was higher than the standard popover z-index of 50 used throughout the application.

Solution

• Changed the z-index from 100 to 40 in webview-ui/src/components/common/CodeBlock.tsx
• This ensures the buttons remain visible and functional above regular content
• They now properly stay below popovers and other overlay elements (z-index: 50)
• The UI layering hierarchy is maintained correctly

Testing

• ✅ All existing CodeBlock tests pass successfully
• ✅ Type checking passes
• ✅ Linting passes
• ✅ Verified no other components have similar high z-index issues

Fixes #7703

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Important

Adjust z-index of CodeBlockButtonWrapper in CodeBlock.tsx to fix overlay issue with popovers.

• Behavior:
  • Adjust z-index of CodeBlockButtonWrapper in CodeBlock.tsx from 100 to 40 to fix overlay issue with popovers.
  • Ensures CodeBlock buttons are below popovers (z-index: 50) but above regular content.
• Testing:
  • All existing tests, type checking, and linting pass.
  • Verified no other components have similar z-index issues.

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[ghost]

Thank you for your contribution! This is a clean and focused fix that correctly addresses the z-index layering issue. The change appropriately reduces the CodeBlock button z-index from 100 to 40, ensuring they stay below popovers (z-index: 50) while remaining above regular content.

The fix is minimal, well-tested, and properly resolves issue #7703. Great work!