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/Contractor/ContractorFormSheet.swift

@@ -284,6 +284,10 @@ struct ContractorFormSheet: View {
                 specialtyPickerSheet
             }
             .onAppear {
+                // Track screen view for new contractors
+                if contractor == nil {
+                    PostHogAnalytics.shared.screen(AnalyticsEvents.newContractorScreenShown)
+                }
                 residenceViewModel.loadMyResidences()
                 loadContractorData()
             }
@@ -493,6 +497,8 @@ struct ContractorFormSheet: View {
 
             viewModel.createContractor(request: request) { success in
                 if success {
+                    // Track contractor creation
+                    PostHogAnalytics.shared.capture(AnalyticsEvents.contractorCreated)
                     onSave()
                     dismiss()
                 }