feat(ui) : add DTOs and services for calendar fields, recurrence, and Zimbra settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/services/dto/zimbra.ts

@@ -0,0 +1,19 @@
+export type ZimbraSettings = {
+    serverUrl: string | null
+    username: string | null
+    calendarPath: string | null
+    enabled: boolean
+    hasPassword: boolean
+}
+
+export type ZimbraSettingsWrite = {
+    serverUrl: string | null
+    username: string | null
+    calendarPath: string | null
+    password?: string | null
+    enabled: boolean
+}
+
+export type ZimbraTestResult = {
+    success: boolean
+}