Improve SCSS cache directory creation and error handling

[shadoath]

Summary

This PR improves the reliability and user experience of the SCSS compiler's cache directory handling by using WordPress's built-in directory creation function and providing more informative error messages.

Key Changes

• Replace mkdir() with wp_mkdir_p(): Changed from the basic PHP mkdir() function to WordPress's wp_mkdir_p() function, which creates directories recursively and respects WordPress conventions
• Enhanced error messaging: Updated error messages to distinguish between two failure scenarios:
  • When the cache directory doesn't exist and cannot be created
  • When the cache directory exists but lacks write permissions
  • This provides clearer guidance to users on how to resolve the issue

Implementation Details

• The wp_mkdir_p() function is more robust than mkdir() as it creates parent directories if needed and handles edge cases better
• The error message now conditionally displays based on whether the cache directory exists, helping users understand the root cause of the permission error
• The cache directory path is included in the error message to help users locate and manually create/fix the directory if needed

https://claude.ai/code/session_01KsZF2sudPbLYyzoWNSjBAG