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/jsMain/kotlin/com/example/casera/analytics/PostHogAnalytics.js.kt

@@ -0,0 +1,31 @@
+package com.example.casera.analytics
+
+/**
+ * JS/Web implementation of PostHog Analytics.
+ * Currently a no-op stub - could be implemented with PostHog JS SDK.
+ */
+actual object PostHogAnalytics {
+    actual fun initialize() {
+        // No-op for web
+    }
+
+    actual fun identify(userId: String, properties: Map<String, Any>?) {
+        // No-op for web
+    }
+
+    actual fun capture(event: String, properties: Map<String, Any>?) {
+        // No-op for web
+    }
+
+    actual fun screen(screenName: String, properties: Map<String, Any>?) {
+        // No-op for web
+    }
+
+    actual fun reset() {
+        // No-op for web
+    }
+
+    actual fun flush() {
+        // No-op for web
+    }
+}