fix(blog): give the prev/next card a radius that exists

--radius-md is not in the vocabulary: the scale is xs/s/m/l/xl/2xl. An
undefined custom property with no fallback makes the declaration invalid at
computed-value time, so border-radius resolved to 0 and the card rendered with
square corners while every card around it is rounded.

.post__seqlink is a bordered surface card in a grid next to PostCard, so it
takes --radius-l (12px), the token the design system documents as the card
radius, and now matches them.

Author: cevheri

src/pages/blog/[...id].astro

@@ -289,7 +289,7 @@ const schema = [
     gap: var(--space-02);
     padding: var(--space-05);
     border: var(--border-width-1) solid var(--border);
-    border-radius: var(--radius-md);
+    border-radius: var(--radius-l);
     background: var(--surface);
   }
   .post__seqlink:hover {