Add PostHog exception capture for crash reporting

Android: uncaught exception handler sends $exception events with stack
trace to PostHog, flushes before delegating to default handler.
iOS: NSSetUncaughtExceptionHandler captures crashes via PostHogSDK,
avoids @MainActor deadlock by calling SDK directly.
Common: captureException() available for non-fatal catches app-wide.
Platform stubs for jvm/js/wasmJs.

composeApp/src/jvmMain/kotlin/com/tt/honeyDue/analytics/PostHogAnalytics.jvm.kt

@@ -17,6 +17,10 @@ actual object PostHogAnalytics {
         // No-op for desktop
     }
 
+    actual fun captureException(throwable: Throwable, properties: Map<String, Any>?) {
+        // No-op for desktop
+    }
+
     actual fun screen(screenName: String, properties: Map<String, Any>?) {
         // No-op for desktop
     }
@@ -28,4 +32,8 @@ actual object PostHogAnalytics {
     actual fun flush() {
         // No-op for desktop
     }
+
+    actual fun setupExceptionHandler() {
+        // No-op for desktop
+    }
 }