fix(11-05): CR-01 surface reminderIsCustom to preserve custom VALARMs on edit

- expand.ts: add reminderIsCustom:boolean to CalendarOccurrence interface;
  derived from classifyValarms kind==='custom'; propagated to both
  non-recurring and recurring occurrence branches
- client.ts: mirror reminderIsCustom on CalendarOccurrence (atomic mirror)
- EventForm.tsx: extend deriveReminderValue to accept isCustom flag;
  returns '__custom__' when true, making the existing D-08 preserve branch
  live — editing a custom-alarm event now omits reminderLeadMinutes from
  the payload so outboxWorker extractValarms keeps the original VALARM
- Fix existing test fixtures (EventForm.test.tsx, EventDetailPopover.test.tsx)
  to include reminderIsCustom:false on all CalendarOccurrence literals

Fixes CAL-14 Pitfall 1: Apple Calendar absolute DATE-TIME / multi-VALARM
alarms no longer silently stripped on any edit round-trip from the PWA.

apps/pwa/src/api/client.ts

@@ -139,6 +139,14 @@ export interface CalendarOccurrence {
    * (atomic mirror, Plan 11-03).
    */
   reminderLeadMinutes: number | null;
+  /**
+   * True when the event's alarm is custom/absolute/multi-VALARM (not reducible to a
+   * single before-event lead). When true, reminderLeadMinutes is always null and the
+   * form must initialize to '__custom__' to preserve the VALARM on edit (CR-01, Plan 11-05).
+   * Mirrors CalendarOccurrence.reminderIsCustom in apps/api/src/broker/expand.ts
+   * (atomic mirror, Plan 11-05).
+   */
+  reminderIsCustom: boolean;
 }
 
 export interface OccurrencesResponse {