fix(i18n): relative-time strings vollständig lokalisiert

relativeTime() in Dashboard, relTime() in WireGuard Clients+Servers und
CertExpiry() im Cluster-Page haben bisher hardcodiertes Deutsch
(„vor 5s", „abgelaufen vor X Tagen") gerendert, unabhängig von der
gewählten UI-Sprache.

Neu: gemeinsame i18n-Keys common.relTime.{Xs,Xm,Xh,Xd,never} in EN+DE,
sowie cluster.{certDaysRemaining,certExpiredDaysAgo}. Alle Helper-
Funktionen nehmen jetzt t() als Parameter und sind damit sprachunabhängig.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

management-ui/src/i18n/locales/en/common.json

@@ -436,6 +436,8 @@
     "renewSelfConfirm": "Re-sign peer cert with the local CA (1 year)? edgeguard-api restart required after.",
     "certRenewedRestartHint": "Cert renewed — please run sudo systemctl restart edgeguard-api.",
     "certRenewFailed": "Cert renewal failed",
+    "certDaysRemaining": "{{n}} days",
+    "certExpiredDaysAgo": "expired {{n}} d ago",
     "removePeerBtn": "Remove",
     "removePeerConfirmTitle": "Really remove peer from the cluster?",
     "removePeerConfirmDesc": "{{fqdn}} is deleted from ha_nodes. Firewall renderer drops its IP from the peer_ipv4 set. On the peer side edgeguard-api keeps running; manually stop it + delete cluster-tls for a full decommission.",
@@ -910,7 +912,14 @@
     "copy": "Copy",
     "copied": "Copied",
     "close": "Close",
-    "refresh": "Refresh"
+    "refresh": "Refresh",
+    "relTime": {
+      "Xs": "{{n}}s ago",
+      "Xm": "{{n}}m ago",
+      "Xh": "{{n}}h ago",
+      "Xd": "{{n}}d ago",
+      "never": "never"
+    }
   },
   "license": {
     "title": "License",