feat(customer): allow changing an already-registered NetBird API token

The per-customer Auto-Updates card only showed the token input when no
token was registered yet, so there was no way to replace one once set
(e.g. rotating it manually). Adds a "Change token" link that reveals the
same input/save flow at any time.

static/lang/de.json

@@ -97,6 +97,7 @@
     "nbuTokenPlaceholder": "Personal Access Token einfügen…",
     "nbuSaveToken": "Prüfen & Speichern",
     "nbuTokenSaved": "Token gespeichert.",
+    "nbuChangeToken": "Token ändern",
     "nbuVersion": "Client-Version",
     "nbuDisabled": "Deaktiviert",
     "nbuLatest": "Neueste Version",