feat: apply back-pill class to story footer back link

Apply the .back-pill class to the story footer back link and add
:not(.back-pill) guard to .story-footer a rule to prevent the accent
color override. This ensures the back pill maintains its design system
styling (border, background, text color) in story footers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WPJztrVGbwic2xTG7G9fjM

themes/intotheeast/templates/story.html.twig

@@ -58,7 +58,7 @@
     {{ page.content|raw }}
 
     <footer class="story-footer">
-        <a href="{{ page.parent().url }}" onclick="if(history.length > 1){ history.back(); return false; }">← Back</a>
+        <a class="back-pill" href="{{ page.parent().url }}" onclick="if(history.length > 1){ history.back(); return false; }">← Back</a>
     </footer>
 </div>