fix(quick-261005-d5d): Kalender-Test bricht bei unerreichbarem Exchange nach 15 s ab

EWS-Aufrufe ueber httpntlm bekommen eine eigene 15-s-Zeitgrenze;
httpreqs timeout greift waehrend des Verbindungsaufbaus nicht
(gemessen 134 s). Netzfehler liefern im Test den Schluessel
'unreachable', das Formular meldet 'nicht erreichbar' statt
'Zugangsdaten pruefen'.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -332,6 +332,7 @@
       "formTesting": "Checking connection …",
       "formTestSuccess": "Connection successful.",
       "formTestFailed": "Connection failed. Please check the address and credentials.",
+      "formTestUnreachable": "The calendar server is not reachable. Please check the address and whether the server can be reached from the Tessera server.",
       "formCancel": "Cancel",
       "formUrlErrorHttps": "The address must start with https://.",
       "formUrlErrorInvalid": "Please enter a valid address.",