Summary

This PR addresses the missing strikethrough formatting option in the markdown editor's visual toolbar. While strikethrough functionality was already available via keyboard shortcut (Ctrl+Shift+S), there was no visual button for users who prefer mouse interactions or are unaware of the hotkey.

Test plan

• Start the development server: npm run start
• Navigate to a markdown editor widget
• Add "strikethrough" to the toolbar buttons configuration
• Verify the strikethrough button appears in the toolbar with proper icon
• Test functionality:
  • Click strikethrough button to apply formatting
  • Click again to remove formatting
  • Verify Ctrl+Shift+S hotkey still works
  • Confirm strikethrough text renders correctly in preview

Preview

Checklist

• I have read the contribution guidelines.