Show layered-protection guide link on every article

Adds a layout-level "Essential reading" block below the article body
that links to the seven-layers pillar in the same language. Hidden on
the pillar article itself. Avoids hand-editing 18 markdown files and
guarantees every future article inherits the link automatically.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

layouts/_default/single.html

@@ -36,6 +36,24 @@
       {{ .Content }}
     </div>
 
+    {{/* Essential reading: link to the layered-protection pillar on every article except itself */}}
+    {{ if ne .Params.translationKey "layers-of-protection" }}
+      {{ $lang := .Page.Language.Lang }}
+      {{ $layers := where (where .Site.AllPages "Params.translationKey" "layers-of-protection") "Lang" $lang }}
+      {{ with index $layers 0 }}
+      <aside style="margin-top:3rem;padding:1.5rem 1.75rem;border-left:3px solid var(--ifk-accent);background:#f8fafc;border-radius:4px;font-family:system-ui,-apple-system,sans-serif">
+        <div style="font-size:0.75rem;text-transform:uppercase;letter-spacing:0.08em;color:#6b7280;font-weight:600;margin-bottom:0.5rem">
+          {{ if eq $lang "de" }}Weiterführend
+          {{ else if eq $lang "fr" }}Lecture essentielle
+          {{ else }}Essential reading{{ end }}
+        </div>
+        <a href="{{ .Permalink }}" style="font-family:var(--ifk-font);font-size:1.15rem;color:var(--ifk-text);text-decoration:none;font-weight:600;line-height:1.3">
+          {{ .Title }} &rarr;
+        </a>
+      </aside>
+      {{ end }}
+    {{ end }}
+
     {{ if or .NextInSection .PrevInSection }}
     <nav style="margin-top:3rem;padding-top:2rem;border-top:1px solid #e5e7eb;display:flex;justify-content:space-between;gap:1rem">
       {{ with .PrevInSection }}