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/dto/create-calendar-source.dto.ts

@@ -41,6 +41,10 @@ export class CreateCalendarSourceDto {
   @IsIn(['ews', 'graph'])
   exchangeMode?: string;
 
+  @IsOptional()
+  @IsString()
+  domain?: string;
+
   @IsOptional()
   @IsHexColor()
   color?: string;

apps/api/src/calendar/dto/test-calendar-source-config.dto.ts

@@ -0,0 +1,33 @@
+import {
+  IsIn,
+  IsOptional,
+  IsString,
+  IsUrl,
+} from 'class-validator';
+
+export class TestCalendarSourceConfigDto {
+  @IsIn(['caldav', 'ics', 'exchange'])
+  type!: string;
+
+  @IsUrl(
+    { protocols: ['https'], require_protocol: true },
+    { message: 'URL must use HTTPS protocol' },
+  )
+  url!: string;
+
+  @IsOptional()
+  @IsString()
+  username?: string;
+
+  @IsOptional()
+  @IsString()
+  password?: string;
+
+  @IsOptional()
+  @IsIn(['ews', 'graph'])
+  exchangeMode?: string;
+
+  @IsOptional()
+  @IsString()
+  domain?: string;
+}

apps/api/src/calendar/dto/update-calendar-source.dto.ts

@@ -45,6 +45,10 @@ export class UpdateCalendarSourceDto {
   @IsIn(['ews', 'graph'])
   exchangeMode?: string;
 
+  @IsOptional()
+  @IsString()
+  domain?: string;
+
   @IsOptional()
   @IsHexColor()
   color?: string;