Fix iCloud sync: chart and next check-in date stale on secondary device (build 32)

- completedMonthKeys: remove UserDefaults-based MonthlyCheckInStore check (not synced via iCloud)
- MonthlyCheckInCard.effectiveLastCheckInDate: use max() instead of ?? to prefer newer synced snapshot date
- MonthlyCheckInView.lastCompletionDate: use max() of local + latest CoreData snapshot via @FetchRequest
- FirebaseService.logPaywallShown: fix parameter key to paywall_trigger (matches GA4 custom dimension)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PortfolioJournal/Services/FirebaseService.swift

@@ -70,7 +70,7 @@ class FirebaseService {
     func logPaywallShown(trigger: String) {
         guard isConfigured else { return }
         Analytics.logEvent("paywall_shown", parameters: [
-            "trigger": trigger
+            "paywall_trigger": trigger
         ])
     }