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

layout/includes/third-party/pjax.pug

@@ -1,4 +1,4 @@
-- var pjaxExclude = 'a:not([target="_blank"])'
+- var pjaxExclude = 'a:not([target="_blank"]):not([data-fancybox])'
 if theme.pjax.exclude
   each val in theme.pjax.exclude
     - pjaxExclude += `:not([href="${val}"])`