add event CRUD actions and recurring event expansion

- Implement full CRUD in MongoEventRepository (findById, findByUserId, findByDateRange, update, delete)
- Extend ChatService to handle create/update/delete actions with dynamic test responses
- Add recurrenceExpander utility using rrule library for RRULE parsing
- Add eventFormatters utility for German-localized week/month overviews
- Add German translations for days and months in shared Constants
- Update client ChatService to support all event actions (action, eventId, updates params)

apps/client/src/services/ChatService.ts

@@ -5,9 +5,18 @@ import {
   ConversationSummary,
   GetMessagesOptions,
   CreateEventDTO,
+  UpdateEventDTO,
+  EventAction,
 } from "@caldav/shared";
 import { ApiClient } from "./ApiClient";
 
+interface ConfirmEventRequest {
+  action: EventAction;
+  event?: CreateEventDTO;
+  eventId?: string;
+  updates?: UpdateEventDTO;
+}
+
 export const ChatService = {
   sendMessage: async (data: SendMessageDTO): Promise<ChatResponse> => {
     return ApiClient.post<ChatResponse>("/chat/message", data);
@@ -16,9 +25,13 @@ export const ChatService = {
   confirmEvent: async (
     conversationId: string,
     messageId: string,
-    event: CreateEventDTO
+    action: EventAction,
+    event?: CreateEventDTO,
+    eventId?: string,
+    updates?: UpdateEventDTO
   ): Promise<ChatResponse> => {
-    return ApiClient.post<ChatResponse>(`/chat/confirm/${conversationId}/${messageId}`, event);
+    const body: ConfirmEventRequest = { action, event, eventId, updates };
+    return ApiClient.post<ChatResponse>(`/chat/confirm/${conversationId}/${messageId}`, body);
   },
 
   rejectEvent: async (