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/repositories/mongo/models/ChatModel.ts

@@ -25,6 +25,7 @@ const EventSchema = new Schema<CreateEventDTO>(
     note: { type: String },
     isRecurring: { type: Boolean },
     recurrenceRule: { type: String },
+    exceptionDates: { type: [String] },
   },
   { _id: false },
 );
@@ -57,6 +58,11 @@ const ProposedChangeSchema = new Schema<ProposedEventChange>(
       type: String,
       enum: ["confirm", "reject"],
     },
+    deleteMode: {
+      type: String,
+      enum: ["single", "future", "all"],
+    },
+    occurrenceDate: { type: String },
   },
   { _id: false },
 );