feat(11-04): add reminderLeadMinutes to CreateEventPayload + CalendarOccurrence

- CalendarOccurrence: required reminderLeadMinutes: number | null (atomic mirror of expand.ts, Plan 11-03)
- CreateEventPayload: optional reminderLeadMinutes?: number | null with absent/null/0/positive contract (D-08)
- Update CalendarOccurrence fixtures in EventForm.test.tsx + EventDetailPopover.test.tsx to include the new required field (reminderLeadMinutes: null)
- pwa tsc --noEmit exits 0

apps/pwa/src/components/EventDetailPopover.test.tsx

@@ -59,6 +59,7 @@ const TIMED_OCCURRENCE: CalendarOccurrence = {
   location: 'Conference Room B',
   description: 'Daily team sync meeting',
   hasRrule: false,
+  reminderLeadMinutes: null,
 };
 
 const OCCURRENCE_WITH_HTML: CalendarOccurrence = {
@@ -86,6 +87,7 @@ const ALLDAY_OCCURRENCE: CalendarOccurrence = {
   location: null,
   description: null,
   hasRrule: false,
+  reminderLeadMinutes: null,
 };
 
 // ── Import component (after mocks are declared) ───────────────────────────────