Add PostHog analytics integration for Android and iOS

Implement comprehensive analytics tracking across both platforms:

Android (Kotlin):
- Add PostHog SDK dependency and initialization in MainActivity
- Create expect/actual pattern for cross-platform analytics (commonMain/androidMain/iosMain/jvmMain/jsMain/wasmJsMain)
- Track screen views: registration, login, residences, tasks, contractors, documents, notifications, profile
- Track key events: user_registered, user_signed_in, residence_created, task_created, contractor_created, document_created
- Track paywall events: contractor_paywall_shown, documents_paywall_shown
- Track sharing events: residence_shared, contractor_shared
- Track theme_changed event

iOS (Swift):
- Add PostHog iOS SDK via SPM
- Create PostHogAnalytics wrapper and AnalyticsEvents constants
- Initialize SDK in iOSApp with session replay support
- Track same screen views and events as Android
- Track user identification after login/registration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

iosApp/iosApp/Residence/ResidenceSharingManager.swift

@@ -89,6 +89,8 @@ class ResidenceSharingManager: ObservableObject {
 
         do {
             try jsonData.write(to: tempURL)
+            // Track residence shared event
+            PostHogAnalytics.shared.capture(AnalyticsEvents.residenceShared, properties: ["method": "file"])
             return tempURL
         } catch {
             print("ResidenceSharingManager: Failed to write .casera file: \(error)")

iosApp/iosApp/Residence/ResidencesListView.swift

@@ -111,6 +111,7 @@ struct ResidencesListView: View {
             }
         }
         .onAppear {
+            PostHogAnalytics.shared.screen(AnalyticsEvents.residenceScreenShown)
             if authManager.isAuthenticated {
                 viewModel.loadMyResidences()
             }