Byline: kein Unterstrich; Link für Standard-Autor robust via E-Mail auflösen

- .byline-author/.journal-author: border-bottom entfernt; Stil nur noch auf
  echten Links (a.…), dezenter Hover statt Unterstrich
- Standard-Autor (ohne .Params.author) wird über site.Params.author.email in
  data/authors aufgelöst → Link greift unabhängig vom Namen, sobald Profil
  gespeichert ist; sonst Fallback auf urlize(Name)

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

layouts/_default/single.html

@@ -16,7 +16,9 @@
         <p class="single-summary">{{ . }}</p>
       {{ end }}
       {{ $author := .Params.author | default site.Params.author.name }}
-      {{ $authorPage := cond (ne $author "") (site.GetPage (printf "/authors/%s" (urlize $author))) false }}
+      {{ $aslug := urlize $author }}
+      {{ if not .Params.author }}{{ with index site.Data.authors site.Params.author.email }}{{ with .slug }}{{ $aslug = . }}{{ end }}{{ end }}{{ end }}
+      {{ $authorPage := and (ne $aslug "") (site.GetPage (printf "/authors/%s" $aslug)) }}
       {{ if or $author .Date }}
         <p class="single-byline">
           {{- with $author -}}