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>

composeApp/src/commonMain/kotlin/com/example/casera/ui/components/AddContractorDialog.kt

@@ -20,6 +20,8 @@ import com.example.casera.models.ContractorUpdateRequest
 import com.example.casera.models.Residence
 import com.example.casera.network.ApiResult
 import com.example.casera.repository.LookupsRepository
+import com.example.casera.analytics.PostHogAnalytics
+import com.example.casera.analytics.AnalyticsEvents
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -53,8 +55,11 @@ fun AddContractorDialog(
     var expandedResidenceMenu by remember { mutableStateOf(false) }
     val contractorSpecialties by LookupsRepository.contractorSpecialties.collectAsState()
 
-    // Load residences for picker
+    // Track screen view and load residences for picker
     LaunchedEffect(Unit) {
+        if (contractorId == null) {
+            PostHogAnalytics.screen(AnalyticsEvents.NEW_CONTRACTOR_SCREEN_SHOWN)
+        }
         residenceViewModel.loadResidences()
     }
 
@@ -91,6 +96,7 @@ fun AddContractorDialog(
 
     LaunchedEffect(createState) {
         if (createState is ApiResult.Success) {
+            PostHogAnalytics.capture(AnalyticsEvents.CONTRACTOR_CREATED)
             onContractorSaved()
             viewModel.resetCreateState()
         }

composeApp/src/commonMain/kotlin/com/example/casera/ui/components/AddTaskDialog.kt

@@ -22,6 +22,8 @@ import com.example.casera.models.TaskCategory
 import com.example.casera.models.TaskCreateRequest
 import com.example.casera.models.TaskFrequency
 import com.example.casera.models.TaskPriority
+import com.example.casera.analytics.PostHogAnalytics
+import com.example.casera.analytics.AnalyticsEvents
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -110,6 +112,11 @@ fun AddTaskDialog(
         showSuggestions = false
     }
 
+    // Track screen view
+    LaunchedEffect(Unit) {
+        PostHogAnalytics.screen(AnalyticsEvents.NEW_TASK_SCREEN_SHOWN)
+    }
+
     // Set defaults when data loads
     LaunchedEffect(frequencies) {
         if (frequencies.isNotEmpty()) {