Fix end-time reminder firing at event start instead of end (ical.net Period.EndTime null bug)

[Copilot]

occurrence.Period.EndTime is always null in ical.net 5.1.1 when using the single-argument GetOccurrences(startArg) overload. The Period wrapper silently fell back to StartTime, so end reminders were scheduled 5 min before the event start rather than its end — and the displayed end time was wrong.

Confirmed against the live calendar: for NWDC（BaaS）with DTSTART=20:30 DTEND=22:30, the end reminder fired at 20:25 instead of 22:25.

Changes

• CalendarExtensions.cs — Period.EndTime: Replace the p.EndTime ?? p.StartTime fallback with a ComputeEndTime() helper that derives end time as occurrence.Start + (ev.End − ev.Start). Using duration rather than ev.End directly is required for recurring events — ev.End carries the base occurrence's date and would be wrong for all future recurrences.

  // Before: silent wrong fallback
  public DateTimeOffset EndTime { get; } = (p.EndTime ?? p.StartTime).ToLocalNetworkTime();
  
  // After: duration-based, correct for both single and recurring events
  private static DateTimeOffset ComputeEndTime(Period p, CalendarEvent? calendarEvent)
  {
      if (p.EndTime is not null) return p.EndTime.ToLocalNetworkTime();
      if (calendarEvent?.Start is not null)
      {
          var duration = (calendarEvent.End?.AsUtc ?? calendarEvent.Start.AsUtc)
                       - calendarEvent.Start.AsUtc;
          var endUtc = p.StartTime.AsUtc.Add(duration);
          return new DateTimeOffset(endUtc).ToOffset(NetworkTime.LocalTimeZoneOffset);
      }
      return p.StartTime.ToLocalNetworkTime();
  }

• CalendarExtensions.cs — GetEvents(): Pass calendarEvent to the Period constructor so it has access to the event duration.

• ReminderTask.cs: End reminder message and log now display eventEndTime (the per-occurrence computed value) instead of ev.End?.ToLocalNetworkTime(). Also fixes the log text "分钟后开始日程" → "分钟后结束日程".

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