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>

.planning/quick/260701-calendar-domain-test-button/PLAN.md

@@ -0,0 +1,23 @@
+---
+slug: calendar-domain-test-button
+created: "2026-07-01"
+status: in-progress
+---
+
+# Calendar: Domain-Feld + Test-Button
+
+Füge Domain-Feld (Exchange) und "Verbindung testen"-Button zur Kalenderquellen-Form hinzu.
+
+## Tasks
+
+- [ ] T1: Prisma schema — `domain String?` zu CalendarSource
+- [ ] T2: Migration ausführen
+- [ ] T3: Create + Update DTOs — `domain?: string` hinzufügen
+- [ ] T4: TestCalendarSourceConfigDto erstellen
+- [ ] T5: CalendarService — domain in SOURCE_SAFE_SELECT, addSource, updateSource; `testConnectionFromConfig` Methode
+- [ ] T6: CalendarController — `POST /calendar/sources/test-config` Endpoint
+- [ ] T7: ExchangeProvider — domain in Interface + EWS WebCredentials
+- [ ] T8: calendar-api.ts (Frontend) — domain in Types + `testSourceConfig` Funktion
+- [ ] T9: calendar-source-form.tsx — Domain-Feld + Test-Button mit Statusanzeige
+- [ ] T10: i18n de.json + en.json — neue Translations
+- [ ] T11: Commit