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/commonMain/kotlin/com/tt/honeyDue/analytics/Analytics.kt

@@ -8,9 +8,11 @@ expect object PostHogAnalytics {
     fun initialize()
     fun identify(userId: String, properties: Map<String, Any>? = null)
     fun capture(event: String, properties: Map<String, Any>? = null)
+    fun captureException(throwable: Throwable, properties: Map<String, Any>? = null)
     fun screen(screenName: String, properties: Map<String, Any>? = null)
     fun reset()
     fun flush()
+    fun setupExceptionHandler()
 }
 
 /**