header: always "Internet for Kids", homepage h1: localized subtitle

Header brand is now hardcoded English across all languages.
Homepage h1 uses subtitle from front matter:
- EN: Protecting Children in a Digital World
- DE: Kinderschutz in der digitalen Welt
- FR: Protéger les enfants dans un monde numérique

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

layouts/_default/home.html

@@ -1,7 +1,7 @@
 {{ define "main" }}
 <article class="prose max-w-3xl mx-auto py-12 px-4">
   <h1 style="font-family:var(--ifk-font);font-size:2.75rem;font-weight:600;line-height:1.15;letter-spacing:-0.01em;margin-bottom:1.5rem">
-    {{ .Title }}
+    {{ with .Params.subtitle }}{{ . }}{{ else }}{{ .Title }}{{ end }}
   </h1>
   <div class="home-content">{{ .Content }}</div>