Add multi-select filters, light/dark theme, and skill ZIP downloads

- Add multi-select dropdown component for all filter fields
- Implement light/dark theme toggle with system preference detection
- Add client-side ZIP download for skills using JSZip
- Include file lists in skills metadata for download feature
- Add title tooltips to multi-select options for long values
- Update all pages with consistent theme toggle in header

website/js/utils.js

@@ -78,6 +78,13 @@ function getGitHubUrl(filePath) {
   return `${REPO_GITHUB_URL}/${filePath}`;
 }
 
+/**
+ * Get raw GitHub URL for a file (for fetching content)
+ */
+function getRawGitHubUrl(filePath) {
+  return `${REPO_BASE_URL}/${filePath}`;
+}
+
 /**
  * Show a toast notification
  */