feat: Network/IP-Verwaltung + Mailguard-Design-Übernahme

Backend:
* Migration 0009_networks: network_interfaces (ethernet|vlan|bond|
  bridge|wireguard, role wan|lan|dmz|mgmt|cluster, parent + vlan_id
  für VLANs) + ip_addresses (interface_id FK, address+prefix, is_vip
  + vip_priority für Cluster-Failover-VIPs).
* Repos services/networkifs + services/ipaddresses + Models +
  Handler /api/v1/network-interfaces (CRUD + /:id/ip-addresses)
  und /api/v1/ip-addresses (CRUD).
* /api/v1/system/interfaces refactored auf Go-natives net.Interfaces()
  statt `ip -j addr show` shell-out — die systemd-Sandbox blockt
  AF_NETLINK auch für Go's runtime, deswegen edgeguard-api.service
  RestrictAddressFamilies um AF_NETLINK ergänzt. Output-Shape
  bleibt identisch (ifindex, ifname, flags[], mtu, link_type,
  address, addr_info[]) — Frontend muss nicht angepasst werden.

Frontend:
* Networks-Page (/networks): "System-discovered Interfaces"
  read-only Tags-Card oben, deklarierte Interfaces unten als
  Tabelle mit Modal-CRUD; Type-Switch zeigt parent+vlan_id-Felder
  bei type=vlan; Role-Tags farbig (wan blau, lan grün, dmz orange,
  mgmt purple, cluster magenta).
* IPAddresses-Page (/ip-addresses): Tabelle pro Interface, VIP-
  Toggle blendet vip_priority-Eingabe ein. Goldenes VIP-Tag in der
  Liste.
* Sidebar erweitert um Networks + IP-Adressen + section-grouping.

Design 1:1 von mail-gateway/management-ui/ übernommen:
* enterprise.css verbatim (Inter-Font via Google CDN statt local
  woff2), Sidebar 240px dunkler Gradient #0B1426→#101D33→#0D1829,
  branding-accent #1677ff für Active-State, abgerundete Cards mit
  shadow-Token, Header weiß mit subtilem backdrop-filter.
* AntD-Theme-Tokens: colorPrimary #0EA5E9, fontSize 13, fontFamily
  'Inter', controlHeight 34, borderRadius 6.
* Layout-Komponenten neu strukturiert: AppLayout/Sidebar/Header
  matchen mailguard-Klassen-Naming (.app-layout, .main-content,
  .sidebar-section, .sidebar-menu-item.active, .header-left, …).
* Sidebar mit 4 Sektionen (Übersicht / Routing / Netzwerk / System)
  + Logo-Header + Versions-Footer.

Live-deployed auf 89.163.205.6: Networks-Endpoint listet eth0
(89.163.205.6/24, MAC bc:24:11:64:29:e8) + lo, frontend zeigt sie
als System-Tags in der Networks-Page.

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

management-ui/src/components/Layout/AppLayout.tsx

@@ -1,25 +1,52 @@
-import { Layout } from 'antd'
-import { Outlet } from 'react-router-dom'
+import { useState } from 'react'
+import { Outlet, useLocation } from 'react-router-dom'
+import { useTranslation } from 'react-i18next'
 
-import Header from './Header'
 import Sidebar from './Sidebar'
+import Header from './Header'
 import UpdateBanner from '../UpdateBanner'
 
-const { Sider, Content } = Layout
+// PAGE_TITLES maps the pathname to an i18n nav key. Header reads
+// this to render "where you are". Empty fallback = app.title.
+const PAGE_TITLES: Record<string, string> = {
+  '/dashboard':     'nav.dashboard',
+  '/domains':       'nav.domains',
+  '/backends':      'nav.backends',
+  '/routing-rules': 'nav.routing',
+  '/networks':      'nav.networks',
+  '/ip-addresses':  'nav.ipAddresses',
+  '/cluster':       'nav.cluster',
+  '/settings':      'nav.settings',
+}
 
 export default function AppLayout() {
+  const [sidebarOpen, setSidebarOpen] = useState(false)
+  const location = useLocation()
+  const { t } = useTranslation()
+
+  const titleKey = Object.entries(PAGE_TITLES)
+    .find(([p]) => location.pathname === p || location.pathname.startsWith(p + '/'))?.[1]
+  const title = titleKey ? t(titleKey) : t('app.title')
+
   return (
-    <Layout style={{ minHeight: '100vh' }}>
-      <Sider width={220} theme="dark">
-        <Sidebar />
-      </Sider>
-      <Layout>
-        <Header />
+    <div className="app-layout">
+      {sidebarOpen && (
+        <div
+          className="sidebar-overlay"
+          onClick={() => setSidebarOpen(false)}
+          aria-hidden="true"
+        />
+      )}
+
+      <Sidebar isOpen={sidebarOpen} onClose={() => setSidebarOpen(false)} />
+
+      <main className="main-content">
+        <Header pageTitle={title} onMenuToggle={() => setSidebarOpen(true)} />
         <UpdateBanner />
-        <Content style={{ padding: '24px', background: '#f5f5f5' }}>
+        <div className="content-area">
           <Outlet />
-        </Content>
-      </Layout>
-    </Layout>
+        </div>
+      </main>
+    </div>
   )
 }