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/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