feat(images): add referrerpolicy attribute for privacy protection

Inject referrerpolicy="no-referrer" to image tags in:
- refactor-content.html: post content images
- sidebar.html: avatar image

Prevents Referer header leakage and enables displaying images
from platforms with referrer-based hotlink protection.

Author: AldenWangExis

_includes/refactor-content.html

@@ -161,6 +161,9 @@
       {% assign _left = _left | append: ' loading="lazy"' %}
     {% endif %}
 
+    <!-- add referrerpolicy for images -->
+    {% assign _left = _left | append: ' referrerpolicy="no-referrer"' %}
+
     {% if page.layout == 'home' %}
       <!-- create the image wrapper -->
       {% assign _wrapper_start = '<div class="preview-img ' | append: _wrapper_class | append: '">' %}

_includes/sidebar.html

@@ -7,7 +7,7 @@
         {%- capture avatar_url -%}
           {% include media-url.html src=site.avatar %}
         {%- endcapture -%}
-        <img src="{{- avatar_url -}}" width="112" height="112" alt="avatar" onerror="this.style.display='none'">
+        <img src="{{- avatar_url -}}" width="112" height="112" alt="avatar" referrerpolicy="no-referrer" onerror="this.style.display='none'">
       {%- endif -%}
     </a>