fix: exclude fancybox links from pjax and refactor series logic

- Add :not([data-fancybox]) to pjaxExclude to prevent fancybox links from being loaded via pjax
- Refactor series grouping to use a shared hexo._seriesGroups object built once
- Improve performance by avoiding repeated post iteration in both tag and helper
- Version bump to 5.6.1

package.json

@@ -1,6 +1,6 @@
 {
   "name": "hexo-theme-butterfly",
-  "version": "5.6.0",
+  "version": "5.6.1",
   "description": "A Simple and Card UI Design theme for Hexo",
   "main": "package.json",
   "scripts": {