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

apps/server/src/ai/utils/eventFormatter.ts

@@ -1,11 +1,12 @@
-import { CalendarEvent } from "@calchat/shared";
+import {
+  CalendarEvent,
+  formatDate,
+  formatTime,
+  formatDateTime,
+} from "@calchat/shared";
 
-// German date/time formatting helpers
-export const formatDate = (d: Date) => d.toLocaleDateString("de-DE");
-export const formatTime = (d: Date) =>
-  d.toLocaleTimeString("de-DE", { hour: "2-digit", minute: "2-digit" });
-export const formatDateTime = (d: Date) =>
-  `${formatDate(d)} ${d.toLocaleTimeString("de-DE")}`;
+// Re-export for backwards compatibility
+export { formatDate, formatTime, formatDateTime };
 
 /**
  * Format a list of events for display in the system prompt.