fix(footer): default nav + tagline when the tenant's CMS footer is empty

The unom tenant's footer doc has no sections yet (the multi-tenant migration
backfilled the old footer to the punktfunk tenant), so the footer rendered as
an empty, near-invisible bar. Render a sensible default (Übersicht/Rechtliches
links + site tagline) when there are no CMS sections; CMS content overrides it
once the unom footer is populated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

messages/de.json

@@ -14,5 +14,11 @@
 	"blog_empty": "Noch keine Beiträge.",
 	"blog_all": "Alle Beiträge",
 	"blog_latest_heading": "Aus dem Blog",
-	"blog_back": "Blog"
+	"blog_back": "Blog",
+
+	"footer_overview": "Übersicht",
+	"footer_home": "Startseite",
+	"footer_legal": "Rechtliches",
+	"footer_imprint": "Impressum",
+	"footer_privacy": "Datenschutz"
 }