ui(wiki): vollbreiter Header wie Library (gleiche max-width), Spalten darunter

Wiki-Header (section-header) spannt jetzt die volle Spalte (grid-column 1/-1)
und die .wiki-Box hat dieselbe max-width/Zentrierung wie der normale Inhalt —
so fluchten Library- und Wiki-Header. Seitenleiste + Inhalt darunter.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

layouts/wiki/single.html

@@ -1,14 +1,13 @@
 {{ define "main" }}
   <div class="wiki">
+    <header class="section-header">
+      <p class="section-rubric">{{ with .Params.group }}{{ . }}{{ else }}Wiki{{ end }}</p>
+      <h1 class="section-title">{{ .Title }}</h1>
+      {{ with .Params.summary }}<p class="section-description">{{ . }}</p>{{ end }}
+    </header>
     <aside class="wiki-side">{{ partial "wiki-nav.html" . }}</aside>
 
     <div class="wiki-page">
-      <header class="section-header">
-        <p class="section-rubric">{{ with .Params.group }}{{ . }}{{ else }}Wiki{{ end }}</p>
-        <h1 class="section-title">{{ .Title }}</h1>
-        {{ with .Params.summary }}<p class="section-description">{{ . }}</p>{{ end }}
-      </header>
-
       {{ $hasToC := .Params.toc | default false }}
       {{ $headers := findRE "<h[2-6]" .Content }}
       {{ if and $hasToC (ge (len $headers) 2) }}