feat(ui): Quick-Toggles, Config-Preview, Dashboard-Alerts, Domain-Detail-Health

Quick-Toggle-Switches (kein Modal nötig) für: Backends, Backend-Server,
DNS-Zonen, DNS-Records, Domains (active), Firewall-Rules, NAT-Rules,
Forward-Proxy ACLs, Routing-Rules.

Dashboard: Alert-Banner für komplett ausgefallene Backends (HAProxy-Stats)
und Domains im Maintenance-Mode.

Domain-Detail: HAProxy-Live-Health-Badge (15s Polling), TLS-Cert
ausstellen/erneuern direkt aus dem Detail, Routing-Rules-Panel inline.

Config-Preview (Settings): alle 4 Generatoren (haproxy, nftables, squid,
unbound) rendern via RenderToString ohne Disk-Write — GET /system/config-preview.

ActionButtons: Viewer-Rolle blendet Delete aus (RBAC-Ergänzung).

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

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

@@ -273,7 +273,7 @@
     "tlsCertExpired": "expired",
     "tlsCertError": "error",
     "tlsCertNone": "missing",
-    "tlsCertNoneHint": "No own TLS cert for this domain — HAProxy serves the bootstrap self-signed cert. Issue one in the SSL tab.",
+    "tlsCertNoneHint": "No own TLS cert for this domain — HAProxy serves the bootstrap self-signed cert. Open the domain and click \"Issue certificate\".",
     "hstsMaxAge": "HSTS max-age (sec)",
     "hstsMaxAgeHint": "How long browsers cache the HTTPS-only mandate. Recommended: 31536000 (1 year).",
     "hstsSubdomains": "includeSubDomains",
@@ -312,7 +312,12 @@
     "actions": "Actions",
     "edit": "Edit",
     "delete": "Delete",
-    "deleteConfirm": "Really delete domain {{name}}?"
+    "deleteConfirm": "Really delete domain {{name}}?",
+    "maintenanceTag": "Maintenance",
+    "rateLimitTag": "Rate limited",
+    "routingRulesTitle": "Routing rules — {{name}}",
+    "routingRulesHint": "Path-prefix → backend mappings for this domain. Lowest priority number wins; unmatched requests go to the primary backend.",
+    "routingRulesEmpty": "No routing rules — all requests go to the primary backend."
   },
   "backends": {
     "title": "Backends",
@@ -330,6 +335,7 @@
     "usedBy": "Used by",
     "noDomain": "no domain",
     "attachedDomains": "Domains",
+    "attachedToOther": "currently: #{{id}}",
     "attachedDomainsHint": "Domains that use this backend as their primary. Selecting domains here reconfigures them directly — same source of truth as the Domain modal's backend picker.",
     "selectDomains": "Select domains",
     "lbAlgo": "Load balancing",
@@ -566,6 +572,9 @@
     "ipv6Hint": "Requires IPv6 to be configured on this server's network interfaces. HAProxy reloads automatically after changing this setting.",
     "ipv6Toggled": "IPv6 setting saved.",
     "ipv6Failed": "IPv6 toggle failed",
+    "configPreviewCardTitle": "Config preview",
+    "configPreviewBtn": "Load preview",
+    "configPreviewHint": "Renders the selected service config from the current DB state. Read-only — nothing is written to disk or reloaded.",
     "passwordCardTitle": "Change admin password",
     "currentPassword": "Current password",
     "newPassword": "New password",
@@ -730,6 +739,8 @@
       "title": "Recent alerts",
       "viewAll": "View all"
     },
+    "downBackendsAlert": "{{count}} backend(s) completely down — no server UP",
+    "maintenanceAlert": "{{count}} domain(s) in maintenance mode",
     "onboardingTitle": "Welcome to EdgeGuard",
     "onboardingIntro": "Fresh box — here are the next steps to route customer traffic:",
     "onboardingStep1": "Create a backend pool (app servers behind HAProxy)",