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/src/calendar/providers/exchange.provider.ts

@@ -24,6 +24,7 @@ export class ExchangeProvider implements CalendarProvider {
       username?: string;
       password?: string;
       exchangeMode?: string | null;
+      domain?: string;
       id: string;
       color?: string | null;
     },
@@ -56,6 +57,7 @@ export class ExchangeProvider implements CalendarProvider {
       username?: string;
       password?: string;
       exchangeMode?: string | null;
+      domain?: string;
       id: string;
     },
   ): Promise<boolean> {
@@ -144,6 +146,7 @@ export class ExchangeProvider implements CalendarProvider {
       url: string;
       username?: string;
       password?: string;
+      domain?: string;
       id: string;
       color?: string | null;
     },
@@ -158,6 +161,7 @@ export class ExchangeProvider implements CalendarProvider {
     service.Credentials = new ews.WebCredentials(
       source.username || '',
       source.password || '',
+      source.domain || undefined,
     );
 
     // CalendarView constructor accepts JS Dates in ews-javascript-api
@@ -210,7 +214,7 @@ export class ExchangeProvider implements CalendarProvider {
    * Tests EWS connection by binding to the calendar folder.
    */
   private async testEwsConnection(
-    source: { url: string; username?: string; password?: string },
+    source: { url: string; username?: string; password?: string; domain?: string },
   ): Promise<boolean> {
     const ews: any = await import('ews-javascript-api');
 
@@ -219,6 +223,7 @@ export class ExchangeProvider implements CalendarProvider {
     service.Credentials = new ews.WebCredentials(
       source.username || '',
       source.password || '',
+      source.domain || undefined,
     );
 
     await ews.Folder.Bind(service, ews.WellKnownFolderName.Calendar);