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

packages/shared/src/models/CalendarEvent.ts

@@ -28,6 +28,7 @@ export interface CreateEventDTO {
   note?: string;
   isRecurring?: boolean;
   recurrenceRule?: string;
+  exceptionDates?: string[]; // For display in proposals
 }
 
 export interface UpdateEventDTO {