KI-AGENT: Briefpapierpflege im Frontend ergänzen

frontend/components/MainNav.vue

@@ -322,6 +322,11 @@ const links = computed(() => {
       to: "/settings/texttemplates",
       icon: "i-heroicons-clipboard-document-list",
     } : null,
+    featureEnabled("settingsLetterheads") ? {
+      label: "Briefpapiere",
+      to: "/settings/letterheads",
+      icon: "i-heroicons-document",
+    } : null,
     featureEnabled("settingsTenant") ? {
       label: "Firmeneinstellungen",
       to: "/settings/tenant",