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/components/EventDetailPopover.test.tsx

@@ -60,6 +60,7 @@ const TIMED_OCCURRENCE: CalendarOccurrence = {
   description: 'Daily team sync meeting',
   hasRrule: false,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
 };
 
 const OCCURRENCE_WITH_HTML: CalendarOccurrence = {
@@ -88,6 +89,7 @@ const ALLDAY_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: false,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
 };
 
 // ── Import component (after mocks are declared) ───────────────────────────────