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/lib/calendar-api.ts

@@ -22,6 +22,7 @@ export interface CalendarSource {
   name: string;
   type: 'caldav' | 'ics' | 'exchange';
   exchangeMode?: string | null;
+  domain?: string | null;
   url: string;
   username?: string | null;
   color?: string | null;
@@ -59,6 +60,7 @@ export interface CreateSourcePayload {
   username?: string;
   password?: string;
   exchangeMode?: string;
+  domain?: string;
   color?: string;
 }
 
@@ -73,6 +75,7 @@ export interface UpdateSourcePayload {
   username?: string;
   password?: string;
   exchangeMode?: string;
+  domain?: string;
   color?: string;
   isVisible?: boolean;
 }
@@ -138,6 +141,28 @@ export async function deleteSource(id: string): Promise<void> {
   if (!res.ok) throw new Error('Failed to delete calendar source');
 }
 
+/**
+ * Test a calendar configuration without saving it.
+ * POST /calendar/sources/test-config
+ */
+export async function testSourceConfig(config: {
+  type: string;
+  url: string;
+  username?: string;
+  password?: string;
+  exchangeMode?: string;
+  domain?: string;
+}): Promise<{ success: boolean; error?: string }> {
+  const res = await fetch(`${API_URL}/calendar/sources/test-config`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    credentials: 'include',
+    body: JSON.stringify(config),
+  });
+  if (!res.ok) throw new Error('Test request failed');
+  return res.json();
+}
+
 /**
  * Test connection to a calendar source.
  * POST /calendar/sources/:id/test