feat(cluster): Seite komplett neu — Blank-Page-Fix + Node-Join-Wizard

Seite war komplett leer wenn die Cluster-API einen Fehler zurückgab
(if(!data) return null). Jetzt: sichtbarer Error-Banner mit Retry,
keine leere Seite mehr.

Neuer 4-Schritt-Wizard "Zweiten Node hinzufügen" immer sichtbar:
  1. Installer-Oneliner (copyable)
  2. Join-Token generieren (Button → POST /cluster/join-tokens)
  3. cluster-join-Befehl inkl. Token inline auf der Seite (kein Modal)
  4. systemctl restart edgeguard-api

Token + CA-Fingerprint + Befehl erscheinen direkt in Schritt 3
nach Token-Generierung — kein separates Modal mehr nötig.
common.retry i18n-Key ergänzt.

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

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

@@ -448,7 +448,19 @@
     "noSelf": "Self-registration in ha_nodes failed — run the setup wizard.",
     "peersTitle": "Peers ({{count}})",
     "singleNodeTitle": "Single-node mode",
-    "singleNodeDesc": "Only this box is known. To add another node: `edgeguard-ctl cluster-join` (coming in phase 3.2).",
+    "singleNodeDesc": "Only this box is known. Follow the steps below to join a second node.",
+    "addNodeTitle": "Add second node to cluster",
+    "step1Title": "1 — Install EdgeGuard on the new host",
+    "step1Desc": "Run as root on the new host (Debian 13, amd64):",
+    "step2Title": "2 — Generate a join token here (on this primary node)",
+    "step2Desc": "Creates a one-time HMAC token valid for 30 minutes. The token authorises the new node to receive a signed cluster TLS certificate.",
+    "step3Title": "3 — Run on the new node",
+    "step3NoToken": "Click \"Generate join token\" above — the full command will appear here.",
+    "step4Title": "4 — Restart edgeguard-api on the new node",
+    "step4Desc": "The service loads the new cluster TLS certificate and starts sending heartbeats. The node appears in the Peers table within 30 seconds.",
+    "apiUnreachable": "Cluster API not reachable",
+    "tokenLabel": "Token",
+    "caFingerprintLabel": "CA fingerprint",
     "driftBanner": "Config drift detected",
     "driftBannerDesc": "One or more peers have a different config hash than this node. Either changes are still in the outbox or a peer was edited directly in the DB. Wait for the outbox to drain or run diagnostics.",
     "col": {
@@ -1069,6 +1081,7 @@
     "download": "Download",
     "copy": "Copy",
     "copied": "Copied",
+    "retry": "Retry",
     "close": "Close",
     "refresh": "Refresh",
     "up": "UP",