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/api/prisma/schema.prisma

@@ -40,6 +40,7 @@ model User {
   updatedAt           DateTime  @updatedAt
   lastLoginAt         DateTime?
   avatarPath          String?
+  accentColor         String?
   passwordResetTokens PasswordResetToken[]
 
   @@index([tenantId])
@@ -156,6 +157,7 @@ model CalendarSource {
   name              String
   type              String    // 'caldav' | 'ics' | 'exchange'
   exchangeMode      String?   // 'ews' | 'graph' — only for exchange type
+  domain            String?   // Exchange EWS only: Windows domain (e.g. COMPANY)
   url               String
   username          String?
   encryptedPassword String?   // AES-256-GCM ciphertext (iv:authTag:ciphertext hex)