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/components/Layout/Sidebar.tsx

@@ -1,4 +1,4 @@
-import { DashboardOutlined, GlobalOutlined } from '@ant-design/icons'
+import { BranchesOutlined, DashboardOutlined, DatabaseOutlined, GlobalOutlined, SettingOutlined } from '@ant-design/icons'
 import { Menu, Typography } from 'antd'
 import { useNavigate, useLocation } from 'react-router-dom'
 import { useTranslation } from 'react-i18next'
@@ -11,6 +11,9 @@ export default function Sidebar() {
   const items = [
     { key: '/dashboard', icon: <DashboardOutlined />, label: t('nav.dashboard') },
     { key: '/domains', icon: <GlobalOutlined />, label: t('nav.domains') },
+    { key: '/backends', icon: <DatabaseOutlined />, label: t('nav.backends') },
+    { key: '/routing-rules', icon: <BranchesOutlined />, label: t('nav.routing') },
+    { key: '/settings', icon: <SettingOutlined />, label: t('nav.settings') },
   ]
 
   return (