feat(web): fuenf Uebersetzungsschluessel fuer URL-Platzhalter im Kalenderquellen-Formular

- widgets.calendar.formFieldUrlPlaceholderEws|Graph|Caldav|Ics + formFieldUrlHintEws in de.json und en.json
- Platzhalter identisch in beiden Sprachen, Hinweistext uebersetzt

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -218,6 +218,11 @@
       "formFieldDomain": "Domain",
       "formFieldDomainHint": "Optional – only needed if your Exchange server expects DOMAIN\\user for sign-in.",
       "formFieldUrl": "Address (URL)",
+      "formFieldUrlPlaceholderEws": "https://mail.firma.de/EWS/Exchange.asmx",
+      "formFieldUrlPlaceholderGraph": "https://graph.microsoft.com/v1.0",
+      "formFieldUrlPlaceholderCaldav": "https://caldav.firma.de/dav/",
+      "formFieldUrlPlaceholderIcs": "https://…/kalender.ics",
+      "formFieldUrlHintEws": "Enter the full EWS address including /EWS/Exchange.asmx – the server name alone is not enough.",
       "formFieldUsername": "Username",
       "formFieldPassword": "Password",
       "formFieldColor": "Color",