fix: correct back to blog button URL with language prefix

Change from .Section | relURL to "/blog/" | relLangURL to ensure
the back button properly includes the language prefix (/de/blog/ or
/en/blog/) instead of just /blog/.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

layouts/blog/single.html

@@ -0,0 +1,35 @@
+{{ define "main" }}
+<article class="container py-5">
+  <div class="row">
+    <div class="col-lg-8 mx-auto">
+      <header class="mb-5">
+        <h1>{{ .Title }}</h1>
+
+        <div class="text-muted mb-3">
+          <time datetime="{{ .Date.Format "2006-01-02" }}">
+            {{ .Date.Format "2. January 2006" }}
+          </time>
+        </div>
+
+        {{ if .Description }}
+          <p class="lead">{{ .Description }}</p>
+        {{ end }}
+      </header>
+
+      <div class="blog-content">
+        {{ .Content }}
+      </div>
+
+      <footer class="mt-5 pt-4 border-top">
+        <a href="{{ "/blog/" | relLangURL }}" class="btn btn-outline-secondary">
+          {{ if eq .Language.Lang "de" }}
+            ← Zurück zum Blog
+          {{ else }}
+            ← Back to Blog
+          {{ end }}
+        </a>
+      </footer>
+    </div>
+  </div>
+</article>
+{{ end }}