fix(icalendar): Implement recursive RRULE value formatting

icalendar.ts

@@ -30,15 +30,25 @@ const RRULE_KEY_MAP: Record<string, string> = {
 };
 
 /**
- * Formats an RRULE value for the string representation.
- * Specifically handles Date objects and nested date objects from ts-ics.
+ * Robustly formats an RRULE value for its string representation.
+ * Handles:
+ * - Arrays: joins elements with commas (recursive)
+ * - Date objects: formats as YYYYMMDDTHHMMSSZ
+ * - Objects: extracts .date, .day, or .value properties (recursive)
+ * - Primitives: stringifies directly
  */
 function formatRRuleValue(v: any): string {
+  if (Array.isArray(v)) {
+    return v.map((item) => formatRRuleValue(item)).join(",");
+  }
   if (v instanceof Date) {
     return v.toISOString().replace(/[-:]/g, "").split(".")[0] + "Z";
   }
-  if (typeof v === "object" && v !== null && v.date instanceof Date) {
-    return v.date.toISOString().replace(/[-:]/g, "").split(".")[0] + "Z";
+  if (typeof v === "object" && v !== null) {
+    const val = v.date || v.day || v.value;
+    if (val !== undefined) {
+      return formatRRuleValue(val);
+    }
   }
   return String(v);
 }