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/utils/rruleHelpers.ts

@@ -0,0 +1,49 @@
+import { rrulestr, Frequency } from "rrule";
+
+export interface ParsedRRule {
+  freq: string; // "YEARLY", "MONTHLY", "WEEKLY", "DAILY", etc.
+  until?: Date;
+  count?: number;
+  interval?: number;
+  byDay?: string[]; // ["MO", "WE", "FR"]
+}
+
+const FREQ_NAMES: Record<Frequency, string> = {
+  [Frequency.YEARLY]: "YEARLY",
+  [Frequency.MONTHLY]: "MONTHLY",
+  [Frequency.WEEKLY]: "WEEKLY",
+  [Frequency.DAILY]: "DAILY",
+  [Frequency.HOURLY]: "HOURLY",
+  [Frequency.MINUTELY]: "MINUTELY",
+  [Frequency.SECONDLY]: "SECONDLY",
+};
+
+/**
+ * Parses an RRULE string and extracts the relevant fields.
+ * Handles both with and without "RRULE:" prefix.
+ */
+export function parseRRule(ruleString: string): ParsedRRule | null {
+  if (!ruleString) {
+    return null;
+  }
+
+  try {
+    // Ensure RRULE: prefix is present
+    const normalized = ruleString.startsWith("RRULE:")
+      ? ruleString
+      : `RRULE:${ruleString}`;
+
+    const rule = rrulestr(normalized);
+    const options = rule.options;
+
+    return {
+      freq: FREQ_NAMES[options.freq] || "UNKNOWN",
+      until: options.until || undefined,
+      count: options.count || undefined,
+      interval: options.interval > 1 ? options.interval : undefined,
+      byDay: options.byweekday?.map((d) => d.toString()) || undefined,
+    };
+  } catch {
+    return null;
+  }
+}