feat: add recurring event deletion with three modes

Implement three deletion modes for recurring events:
- single: exclude specific occurrence via EXDATE mechanism
- future: set RRULE UNTIL to stop future occurrences
- all: delete entire event series

Changes include:
- Add exceptionDates field to CalendarEvent model
- Add RecurringDeleteMode type and DeleteRecurringEventDTO
- EventService.deleteRecurring() with mode-based logic using rrule library
- EventController DELETE endpoint accepts mode/occurrenceDate query params
- recurrenceExpander filters out exception dates during expansion
- AI tools support deleteMode and occurrenceDate for proposed deletions
- ChatService.confirmEvent() handles recurring delete modes
- New DeleteEventModal component for unified delete confirmation UI
- Calendar screen integrates modal for both recurring and non-recurring events

apps/server/src/utils/recurrenceExpander.ts

@@ -71,10 +71,19 @@ export function expandRecurringEvents(
         true, // inclusive
       );
 
+      // Build set of exception dates for fast lookup
+      const exceptionSet = new Set(event.exceptionDates || []);
+
       for (const occurrence of occurrences) {
         const occurrenceStart = fromRRuleDate(occurrence);
         const occurrenceEnd = new Date(occurrenceStart.getTime() + duration);
 
+        // Skip if this occurrence is in the exception dates
+        const dateKey = formatDateKey(occurrenceStart);
+        if (exceptionSet.has(dateKey)) {
+          continue;
+        }
+
         expanded.push({
           ...event,
           occurrenceStart,
@@ -113,3 +122,11 @@ function formatRRuleDateString(date: Date): string {
   const seconds = String(date.getSeconds()).padStart(2, "0");
   return `${year}${month}${day}T${hours}${minutes}${seconds}`;
 }
+
+// Format date as YYYY-MM-DD for exception date comparison
+function formatDateKey(date: Date): string {
+  const year = date.getFullYear();
+  const month = String(date.getMonth() + 1).padStart(2, "0");
+  const day = String(date.getDate()).padStart(2, "0");
+  return `${year}-${month}-${day}`;
+}