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/EventForm.test.tsx

@@ -125,6 +125,7 @@ const EDIT_OCCURRENCE: CalendarOccurrence = {
   description: 'Weekly sync',
   hasRrule: false,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
 };
 
 // ── Import component (after mocks) ────────────────────────────────────────────
@@ -511,6 +512,7 @@ const RECURRING_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: true,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
   // @ts-expect-error — recurrence is not on CalendarOccurrence type yet; the reset
   // effect reads it if present and defaults to 'none' when absent (WR-03, v1 comment)
   recurrence: 'weekly',
@@ -538,6 +540,7 @@ const LATE_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: false,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
 };
 
 describe('EventForm — Plan 03-12 gap closures', () => {
@@ -786,6 +789,7 @@ const ALL_DAY_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: false,
   reminderLeadMinutes: null,
+  reminderIsCustom: false,
 };
 
 describe('EventForm — Plan 06-06 end-tracking + recurrence-bound', () => {
@@ -1007,6 +1011,7 @@ const TIMED_REMINDER_OCCURRENCE: CalendarOccurrence = {
   title: 'Meeting with reminder',
   start: '2026-06-15T10:00:00-04:00',
   end: '2026-06-15T11:00:00-04:00',
+  reminderIsCustom: false,
   allDay: false,
   location: null,
   description: null,
@@ -1034,6 +1039,7 @@ const ALLDAY_REMINDER_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: false,
   reminderLeadMinutes: 1440,
+  reminderIsCustom: false,
 };
 
 /**
@@ -1056,6 +1062,7 @@ const OFFLIST_REMINDER_OCCURRENCE: CalendarOccurrence = {
   description: null,
   hasRrule: false,
   reminderLeadMinutes: 45,
+  reminderIsCustom: false,
 };
 
 describe('EventForm — Phase 11 reminder picker (Plan 04)', () => {
@@ -1315,10 +1322,8 @@ describe('EventForm — Phase 11 Plan 05 CR-01: custom alarm round-trip', () =>
       const callPayload = mockUpdateEvent.mock.calls[0][1] as Record<string, unknown>;
       // MUST be absent: the presence of reminderLeadMinutes:null would cause the outbox
       // worker to clear the VALARM — the CR-01 data-loss bug.
-      expect(Object.prototype.hasOwnProperty.call(callPayload, 'reminderLeadMinutes')).toBe(
-        false,
-        'reminderLeadMinutes must be absent from payload when alarm is custom (D-08 preserve path)',
-      );
+      // D-08: field must be absent — presence of reminderLeadMinutes:null clears the VALARM (CR-01)
+      expect(Object.prototype.hasOwnProperty.call(callPayload, 'reminderLeadMinutes')).toBe(false);
     });
   });
 });