feat(calendar): add domain field and test-connection button to Exchange sources

- Prisma: domain String? added to CalendarSource model (db push applied)
- DTOs: domain in CreateCalendarSourceDto, UpdateCalendarSourceDto, new TestCalendarSourceConfigDto
- Service: domain in SOURCE_SAFE_SELECT, addSource, updateSource; new testConnectionFromConfig method
- Controller: POST /calendar/sources/test-config (before :id routes to avoid collision)
- ExchangeProvider: domain in all source interfaces; passed as 3rd arg to EWS WebCredentials
- Frontend: domain in CalendarSource/CreateSourcePayload/UpdateSourcePayload; testSourceConfig API fn
- Form: domain field (Exchange-only), "Test connection" button with idle/loading/success/error states
- i18n: de+en keys for formFieldDomain, formFieldDomainHint, formTestConnection, formTesting, formTestSuccess, formTestFailed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/messages/de.json

@@ -202,6 +202,12 @@
       "formFieldPassword": "Passwort",
       "formFieldColor": "Farbe",
       "formFieldExchangeMode": "Exchange-Modus",
+      "formFieldDomain": "Domain",
+      "formFieldDomainHint": "Windows-Domain (z.B. FIRMA)",
+      "formTestConnection": "Verbindung testen",
+      "formTesting": "Teste...",
+      "formTestSuccess": "Verbindung erfolgreich",
+      "formTestFailed": "Verbindung fehlgeschlagen",
       "formSave": "Speichern",
       "formSaving": "Wird gespeichert...",
       "formCancel": "Abbrechen",