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)

CLAUDE.md

@@ -135,7 +135,9 @@ src/
 │   └── ClaudeAdapter.ts      # Implements AIProvider
 └── utils/
     ├── jwt.ts                # signToken(), verifyToken()
-    └── password.ts           # hash(), compare()
+    ├── password.ts           # hash(), compare()
+    ├── eventFormatters.ts    # getWeeksOverview(), getMonthOverview() - formatted event listings
+    └── recurrenceExpander.ts # expandRecurringEvents() - expand recurring events into occurrences
 ```
 
 **API Endpoints:**
@@ -169,7 +171,8 @@ src/
 │   ├── ChatMessage.ts    # ChatMessage, Conversation, SendMessageDTO, CreateMessageDTO,
 │   │                     # GetMessagesOptions, ChatResponse, ConversationSummary,
 │   │                     # ProposedEventChange, EventAction
-│   └── Constants.ts      # DAYS, MONTHS, Day, Month, DAY_INDEX
+│   └── Constants.ts      # DAYS, MONTHS, Day, Month, DAY_INDEX, DAY_INDEX_TO_DAY,
+│                         # DAY_TO_GERMAN, DAY_TO_GERMAN_SHORT, MONTH_TO_GERMAN
 └── utils/
     ├── index.ts
     └── dateHelpers.ts    # getDay() - get date for specific weekday relative to today
@@ -269,26 +272,28 @@ MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
   - `utils/jwt`: signToken() (verifyToken() pending)
   - `dotenv` integration for environment variables
   - `ChatController`: sendMessage(), confirmEvent(), rejectEvent()
-  - `ChatService`: processMessage() with test responses, confirmEvent() saves events to DB, rejectEvent()
+  - `ChatService`: processMessage() with test responses (create, update, delete actions), confirmEvent() handles all CRUD actions
+  - `MongoEventRepository`: Full CRUD implemented (findById, findByUserId, findByDateRange, create, update, delete)
+  - `utils/eventFormatters`: getWeeksOverview(), getMonthOverview() with German localization
+  - `utils/recurrenceExpander`: expandRecurringEvents() using rrule library for RRULE parsing
 - **Stubbed (TODO):**
   - `AuthMiddleware.authenticate()`: Currently uses fake user for testing
   - `AuthController`: refresh(), logout()
   - `AuthService`: refreshToken()
   - `ChatController`: getConversations(), getConversation()
   - `MongoChatRepository`: Database persistence for chat
-  - `MongoEventRepository`: Only create() implemented, rest stubbed
 - **Not started:**
   - `EventController`, `EventService`
   - `ClaudeAdapter` (AI integration - currently using test responses)
 
-**Shared:** Types, DTOs, constants (Day, Month), and date utilities defined and exported.
+**Shared:** Types, DTOs, constants (Day, Month with German translations), and date utilities defined and exported.
 
 **Frontend:**
 - Tab navigation (Chat, Calendar) implemented with basic UI
 - Calendar screen has month navigation and grid display (partially functional)
 - Chat screen functional with FlashList, message sending, and event confirm/reject
 - `ApiClient`: get(), post() implemented
-- `ChatService`: sendMessage(), confirmEvent(convId, msgId, event), rejectEvent() - confirmEvent sends CreateEventDTO in body
+- `ChatService`: sendMessage(), confirmEvent(convId, msgId, action, event?, eventId?, updates?), rejectEvent() - supports create/update/delete actions
 - `ProposedEventCard`: Displays proposed events (title, date, description, recurring indicator) with confirm/reject buttons
 - `Themes.tsx`: Centralized color definitions including button colors
 - Auth screens (Login, Register), Event Detail, and Note screens exist as skeletons