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/css/style.css

@@ -1183,7 +1183,7 @@ body::after {
     padding-top: var(--space-8);
     border-top: 1px solid var(--color-border);
 }
-.story-footer a {
+.story-footer a:not(.back-pill) {
     font-family: var(--font-ui);
     font-size: var(--text-sm);
     color: var(--color-accent);