Extract shared CSS into separate style block with validation

[namingbe]

Summary

This PR introduces a mechanism to maintain consistent shared CSS across multiple HTML files by extracting common styles into a dedicated <style data-shared> block and adding automated validation via GitHub Actions.

Key Changes

• Added GitHub Actions workflow (check-shared-css.yml): Automatically verifies that all HTML files with <style data-shared> blocks contain identical CSS, preventing style drift across the codebase
• Refactored CSS in index.html and playground.html:
  • Moved common CSS variables and universal reset rules (*, body base styles) into a shared <style data-shared> block
  • Separated page-specific CSS variables (--add, --del, --drop-active) into a separate <style> block to maintain file-specific customizations
  • Preserved all existing styles and functionality

Implementation Details

• The shared CSS block includes foundational styles: CSS custom properties (colors, fonts), universal selector reset, and base body styling
• Page-specific variables remain in separate style blocks, allowing each page to customize behavior while maintaining a consistent visual foundation
• The validation workflow uses xmllint to extract and compare data-shared style blocks, ensuring consistency is maintained as the codebase evolves
• The workflow requires at least 2 files with shared CSS blocks to be present, preventing accidental removal of the feature

https://claude.ai/code/session_01S3ETpFXfDZUnDPky8Vs6Gw