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/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",