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/en.json

@@ -97,6 +97,7 @@
     "nbuTokenPlaceholder": "Paste Personal Access Token…",
     "nbuSaveToken": "Verify & Save",
     "nbuTokenSaved": "Token saved.",
+    "nbuChangeToken": "Change token",
     "nbuVersion": "Client version",
     "nbuDisabled": "Disabled",
     "nbuLatest": "Latest version",