init: scaffold OPENBUREAU site with shibui-derived theme

- Hugo site for openbureau.ch (Deutsch, i18n-ready for EN/IT)
- Theme themes/openbureau/ = local copy of shibui, customized via
  site-level layouts and assets to keep the theme reference clean
- Editorial typography stack: Newsreader serif body, Space Grotesk
  display, Inter for listings, IBM Plex Mono for technical meta
- Content structure: library/ (Theorie, Büroführung, Software) with
  manifest and colophon at root; software is a library category, not
  a separate top-level
- Three views over one source: Journal (chronological home),
  Library (atlas grouped by section + tag cloud), single articles

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

themes/openbureau/layouts/404.html

@@ -0,0 +1,24 @@
+{{ define "main" }}
+  <h1>RESOURCE NOT FOUND</h1>
+  <p>the page you're looking for doesn't exist or has been moved.</p>
+  
+  <nav aria-label="404 navigation">
+    <ul>
+      <li><a href="{{ "/" | relURL }}">← return to homepage</a></li>
+      {{ with site.Home }}
+        {{ with .GetPage "posts" }}
+          {{ if . }}
+            <li><a href="{{ .RelPermalink }}">browse posts</a></li>
+          {{ end }}
+        {{ end }}
+      {{ end }}
+      {{ with site.Home }}
+        {{ with .GetPage "about" }}
+          {{ if . }}
+            <li><a href="{{ .RelPermalink }}">about page</a></li>
+          {{ end }}
+        {{ end }}
+      {{ end }}
+    </ul>
+  </nav>
+{{ end }}