fix(sass): correct RFS vendor import path for GitHub Actions

- Change vendor/rfs import to bootstrap/vendor/rfs
- Fixes build error in GitHub Actions CI environment
- Resolves "File to import not found or unreadable: vendor/rfs"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mrleeio

_sass/bootstrap/_mixins.scss

@@ -3,7 +3,7 @@
 // Used in conjunction with global variables to enable certain theme features.
 
 // Vendor
-@import "vendor/rfs";
+@import "bootstrap/vendor/rfs";
 
 // Deprecate
 @import "mixins/deprecate";