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.tsx

@@ -75,12 +75,22 @@ function humanizeReminderLead(minutes: number): string {
 }
 
 /**
- * Derive the initial reminder picker value from an occurrence's reminderLeadMinutes.
- * Returns '__none__' for null, the matching preset string for a preset, or the numeric
- * string for an off-list value (synthetic option will be rendered for this case).
- * There is no '__custom__' path here — the occurrence only carries number|null.
+ * Derive the initial reminder picker value from an occurrence's reminder fields.
+ *
+ * CR-01 (Plan 11-05): When reminderIsCustom is true, the event carries an absolute
+ * DATE-TIME trigger or multiple VALARMs that cannot be reduced to a single lead.
+ * Returning '__custom__' makes the existing preserve branch live: the form emits an
+ * absent reminderLeadMinutes field, and the outboxWorker's D-08 preserve path keeps
+ * the original VALARM intact (no silent data loss on edit).
+ *
+ * Precedence: custom → '__custom__'; null → '__none__'; else preset/off-list string.
  */
-function deriveReminderValue(leadMinutes: number | null, isAllDay: boolean): string {
+function deriveReminderValue(
+  leadMinutes: number | null,
+  isAllDay: boolean,
+  isCustom = false,
+): string {
+  if (isCustom) return '__custom__';
   if (leadMinutes === null) return '__none__';
   const presets = isAllDay ? ALLDAY_REMINDER_PRESETS : TIMED_REMINDER_PRESETS;
   if (presets.has(leadMinutes)) return String(leadMinutes);
@@ -317,8 +327,13 @@ export function EventForm() {
       setLocation(occurrence?.location ?? '');
       setDescription(occurrence?.description ?? '');
       // Phase 11: derive reminder picker value from occurrence (edit-mode pre-population, D-01/D-07)
+      // CR-01 (Plan 11-05): pass reminderIsCustom so custom alarms initialize to '__custom__'
+      // instead of '__none__', making the D-08 preserve path reachable on edit.
       const occAllDay = occurrence?.allDay ?? false;
-      setReminderValue(deriveReminderValue(occurrence?.reminderLeadMinutes ?? null, occAllDay));
+      const occIsCustom = occurrence?.reminderIsCustom ?? false;
+      setReminderValue(
+        deriveReminderValue(occurrence?.reminderLeadMinutes ?? null, occAllDay, occIsCustom),
+      );
     }
   }, [eventFormOpen, eventFormMode, eventFormUid, occurrence?.uid]); // eslint-disable-line react-hooks/exhaustive-deps