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

package-lock.json

@@ -12535,7 +12535,10 @@
     },
     "packages/shared": {
       "name": "@calchat/shared",
-      "version": "1.0.0"
+      "version": "1.0.0",
+      "dependencies": {
+        "rrule": "^2.8.1"
+      }
     }
   }
 }