feat(ui): (a) Backends + Routing-Rules + Settings pages + Sidebar

CRUD-Pages analog Domains:
* Backends: AntD Table + Modal-Form mit name, scheme, address, port,
  health_check_path, active. TanStack-Query gegen /api/v1/backends.
* RoutingRules: Table mit Domain-Name- und Backend-Label-Resolution,
  Modal mit Select-Pickern für Domain + Backend, Path-Prefix,
  Priority, Active. Drei parallele Queries (rules, domains, backends)
  liefern die Listen.
* Settings: read-only Descriptions-Cards mit /system/health und
  /setup/status. Editable Werte folgen später.

Sidebar erweitert um Backends, Routing-Rules, Settings (mit AntD-
Icons). i18n de/en für alle drei neuen Seiten.

bun run build + npx tsc -b strict (0 errors). Live-Smoke gegen API:
SPA-Routes /backends, /routing-rules, /settings antworten 200 mit
index.html (NoRoute fallback wirkt).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -55,6 +55,45 @@
     "delete": "Delete",
     "deleteConfirm": "Really delete domain {{name}}?"
   },
+  "backends": {
+    "title": "Backends",
+    "addBackend": "Add backend",
+    "editBackend": "Edit backend",
+    "name": "Name",
+    "scheme": "Scheme",
+    "address": "Address",
+    "port": "Port",
+    "target": "Target",
+    "healthCheck": "Health check path",
+    "active": "Active",
+    "actions": "Actions",
+    "deleteConfirm": "Really delete backend {{name}}?"
+  },
+  "routing": {
+    "title": "Routing rules",
+    "addRule": "Add rule",
+    "editRule": "Edit rule",
+    "domain": "Domain",
+    "pathPrefix": "Path prefix",
+    "backend": "Backend",
+    "priority": "Priority",
+    "active": "Active",
+    "actions": "Actions",
+    "selectDomain": "Select domain",
+    "selectBackend": "Select backend",
+    "deleteConfirm": "Really delete this routing rule?"
+  },
+  "settings": {
+    "title": "Settings",
+    "intro": "System information and setup status. Editable values come in a later release.",
+    "systemInfo": "System",
+    "version": "Version",
+    "status": "Status",
+    "setupInfo": "Setup",
+    "adminEmail": "Admin email",
+    "fqdn": "FQDN",
+    "setupCompleted": "Setup completed"
+  },
   "update": {
     "available": "Update available: {{pkg}} {{installed}} → {{available}}",
     "applyNow": "Apply now",
@@ -67,6 +106,8 @@
     "save": "Save",
     "cancel": "Cancel",
     "loading": "Loading …",
-    "error": "Error"
+    "error": "Error",
+    "edit": "Edit",
+    "delete": "Delete"
   }
 }