fix(i18n): hardkodierte Strings in Cluster/Users/Backups/Alerts bereinigen

- Cluster: statusTag() bekommt t-Parameter; Status-Labels (online/offline/
  joining/leaving/unknown) → cluster.status.* Keys
- Users: 'You'-Tag → t('users.you') / de: 'Ich'
- Backups: 'OK'-Status-Tag → t('backups.okTag')
- Alerts: Kanal-Kind-Labels + alle SMTP-Formular-Labels in t() überführt
- en/de: cluster.status.*, users.you, backups.okTag, alerts.smtp.*,
  alerts.kindWebhook/kindEmail ergänzt

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

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

@@ -454,7 +454,14 @@
     "joinTokenOneShot": "Shown once, usable once",
     "joinTokenOneShotDesc": "Transfer the token securely now — it won't be shown again. Valid until {{expires}}. Server marks it as consumed when redeemed via cluster-join.",
     "joinTokenFailed": "Token generation failed",
-    "joinCmdLabel": "Run on the new node:"
+    "joinCmdLabel": "Run on the new node:",
+    "status": {
+      "online": "online",
+      "offline": "offline",
+      "joining": "joining",
+      "leaving": "leaving",
+      "unknown": "unknown"
+    }
   },
   "ssl": {
     "title": "SSL certificates",
@@ -1048,6 +1055,16 @@
     "emptyEventsDesc": "Triggers (cert expiry, backup failure, cluster drift, license invalid, etc.) haven't fired any events yet. When they do, they land here and get delivered to the configured channels.",
     "noChannels": "no active channel",
     "confirmDelete": "Really delete channel {{name}}?",
+    "kindWebhook": "Webhook (Slack/Discord/Teams/HTTP endpoint)",
+    "kindEmail": "Email (SMTP)",
+    "smtp": {
+      "host": "SMTP host",
+      "port": "SMTP port",
+      "from": "From",
+      "username": "Username",
+      "password": "Password",
+      "starttls": "STARTTLS"
+    },
     "col": {
       "name": "Name",
       "kind": "Kind",
@@ -1132,6 +1149,7 @@
     "confirmDelete": "Really delete backup {{file}}?",
     "empty": "No backups yet. Click “Run backup now” or wait for the next scheduled tick.",
     "failedTag": "FAILED",
+    "okTag": "OK",
     "col": {
       "time": "Time",
       "file": "File",
@@ -1226,6 +1244,7 @@
     "emptyDesc": "Add the first additional user above.",
     "errorEmailTaken": "This email address is already in use.",
     "cannotDeleteSelf": "You cannot delete your own account.",
+    "you": "You",
     "never": "Never"
   },
   "audit": {