feat: add RRULE parsing to shared package and improve ProposedEventCard UI

- Add rrule library to shared package for RRULE string parsing
- Add rruleHelpers.ts with parseRRule() returning freq, until, count, interval, byDay
- Add formatters.ts with German date/time formatters for client and server
- Extend CreateEventDTO with exceptionDates field for proposals
- Extend ChatModel schema with exceptionDates, deleteMode, occurrenceDate
- Update proposeUpdateEvent tool to support isRecurring and recurrenceRule params
- ProposedEventCard now shows green "Neue Ausnahme" and "Neues Ende" text
- Add Sport test scenario with dynamic exception and UNTIL responses
- Update CLAUDE.md documentation

CLAUDE.md

@@ -309,7 +309,9 @@ src/
 │                         # DAY_TO_GERMAN, DAY_TO_GERMAN_SHORT, MONTH_TO_GERMAN
 └── utils/
     ├── index.ts
-    └── dateHelpers.ts    # getDay() - get date for specific weekday relative to today
+    ├── dateHelpers.ts    # getDay() - get date for specific weekday relative to today
+    ├── formatters.ts     # formatDate(), formatTime(), formatDateTime(), formatDateWithWeekday() - German locale
+    └── rruleHelpers.ts   # parseRRule() - parse RRULE strings to extract freq, until, count, interval, byDay
 ```
 
 **Key Types:**
@@ -326,7 +328,7 @@ src/
 - `Conversation`: id, userId, createdAt?, updatedAt? (messages loaded separately via lazy loading)
 - `CreateUserDTO`: email, userName, password (for registration)
 - `LoginDTO`: identifier (email OR userName), password
-- `CreateEventDTO`: Used for creating events AND for AI-proposed events
+- `CreateEventDTO`: Used for creating events AND for AI-proposed events, includes optional `exceptionDates` for proposals
 - `GetMessagesOptions`: Cursor-based pagination with `before?: string` and `limit?: number`
 - `ConversationSummary`: id, lastMessage?, createdAt? (for conversation list)
 - `UpdateMessageDTO`: proposalId?, respondedAction? (for marking individual proposals as confirmed/rejected)
@@ -467,7 +469,8 @@ NODE_ENV=development        # development = pretty logs, production = JSON
   - `ChatRepository` interface: updateMessage() and updateProposalResponse() for per-proposal respondedAction tracking
   - `GPTAdapter`: Full implementation with OpenAI GPT (gpt-4o-mini model), function calling for calendar operations, collects multiple proposals per response
   - `ai/utils/`: Provider-agnostic shared utilities (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
-  - `ai/utils/systemPrompt`: Includes RRULE documentation - AI knows to create separate events when times differ by day
+  - `ai/utils/systemPrompt`: Includes RRULE documentation - AI knows to create separate events when times differ by day, warns AI not to put RRULE in description field
+  - `ai/utils/toolDefinitions`: proposeUpdateEvent supports `isRecurring` and `recurrenceRule` parameters for adding UNTIL or modifying recurrence
   - `utils/recurrenceExpander`: Handles RRULE parsing, strips `RRULE:` prefix if present (AI may include it), filters out exceptionDates
   - `logging/`: Structured logging with pino, pino-http middleware, @Logged decorator
   - All repositories and GPTAdapter decorated with @Logged for automatic method logging
@@ -477,7 +480,11 @@ NODE_ENV=development        # development = pretty logs, production = JSON
   - `AuthService`: refreshToken()
   - JWT authentication (currently using simple X-User-Id header)
 
-**Shared:** Types, DTOs, constants (Day, Month with German translations), ExpandedEvent type, and date utilities defined and exported.
+**Shared:**
+- Types, DTOs, constants (Day, Month with German translations), ExpandedEvent type defined and exported
+- `rruleHelpers.ts`: `parseRRule()` parses RRULE strings using rrule library, returns `ParsedRRule` with freq, until, count, interval, byDay
+- `formatters.ts`: German date/time formatters (`formatDate`, `formatTime`, `formatDateTime`, `formatDateWithWeekday`) used by both client and server
+- rrule library added as dependency for RRULE parsing
 
 **Frontend:**
 - **Authentication fully implemented:**
@@ -525,7 +532,7 @@ NODE_ENV=development        # development = pretty logs, production = JSON
 - `ModalBase`: Reusable modal wrapper with backdrop (absolute-positioned behind card), uses CardBase internally - provides click-outside-to-close, Android back button support, and proper scrolling on Android
 - `EventCardBase`: Event card with date/time/recurring icons - uses CardBase for structure
 - `EventCard`: Uses EventCardBase + edit/delete buttons (TouchableOpacity with delayPressIn for scroll-friendly touch handling)
-- `ProposedEventCard`: Uses EventCardBase + confirm/reject buttons for chat proposals (supports create/update/delete actions with deleteMode display)
+- `ProposedEventCard`: Uses EventCardBase + confirm/reject buttons for chat proposals, displays green highlighted text for new changes ("Neue Ausnahme: [date]" for single delete, "Neues Ende: [date]" for UNTIL updates)
 - `DeleteEventModal`: Delete confirmation modal using ModalBase - shows three options for recurring events (single/future/all), simple confirm for non-recurring
 - `EventOverlay` (in calendar.tsx): Day events overlay using ModalBase - shows EventCards for selected day
 - `Themes.tsx`: Theme definitions with THEMES object (defaultLight, defaultDark) including all color tokens (textPrimary, borderPrimary, eventIndicator, secondaryBg, shadowColor, etc.)