fix(i18n): WireGuard QR/.conf Buttons + Backend 'Upgrade gestartet' → Englisch

- Servers.tsx: 'QR' und '.conf' Button-Labels in t() überführt (wg.peer.qr/conf)
- system.go: hardkodierter deutscher API-Response-String 'Upgrade gestartet'
  → 'upgrade started' (API-Responses sollen immer Englisch sein)

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

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

@@ -692,6 +692,8 @@
       "pskOn": "PSK generieren",
       "pskOff": "kein PSK",
       "downloadConf": "wg-quick.conf herunterladen",
+      "qr": "QR",
+      "conf": ".conf",
       "qrTitle": "WireGuard-QR",
       "qrHint": "Mit der WireGuard-App (iOS/Android) scannen: \"Tunnel hinzufügen\" → \"QR-Code scannen\". Endpoint im Download-Conf bitte vor Verwendung anpassen.",
       "online": "Online",

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

@@ -692,6 +692,8 @@
       "pskOn": "Generate PSK",
       "pskOff": "no PSK",
       "downloadConf": "Download wg-quick.conf",
+      "qr": "QR",
+      "conf": ".conf",
       "qrTitle": "WireGuard QR",
       "qrHint": "Scan with the WireGuard app (iOS/Android): \"Add tunnel\" → \"Scan QR code\". Replace the Endpoint placeholder in the downloaded conf before use.",
       "online": "Online",

management-ui/src/pages/Wireguard/Servers.tsx

@@ -443,7 +443,7 @@ function PeerDrawer({ iface, onClose }: PeerDrawerProps) {
         <Space size={4}>
           {row.has_private_key && (
             <>
-              <Button size="small" type="text" icon={<QrcodeOutlined />} onClick={() => setQrPeer(row)}>QR</Button>
+              <Button size="small" type="text" icon={<QrcodeOutlined />} onClick={() => setQrPeer(row)}>{t('wg.peer.qr')}</Button>
               <Button
                 size="small"
                 type="text"
@@ -451,7 +451,7 @@ function PeerDrawer({ iface, onClose }: PeerDrawerProps) {
                 href={`/api/v1/wireguard/peers/${row.id}/config`}
                 target="_blank"
                 title={t('wg.peer.downloadConf')}
-              >.conf</Button>
+              >{t('wg.peer.conf')}</Button>
             </>
           )}
           <ActionButtons