fix(ui): Wire system back gesture to in-app navigation

Add PlatformBackHandler (expect/actual) to intercept the back gesture
on Android via BackHandler, and enable OnBackInvokedCallback in the
manifest so the swipe-back gesture triggers onBack instead of exiting
the app.

Fixes the back gesture exiting to home from the event detail screen.

composeApp/src/androidMain/AndroidManifest.xml

@@ -7,6 +7,7 @@
         android:label="MyIce"
         android:icon="@mipmap/ic_launcher"
         android:supportsRtl="true"
+        android:enableOnBackInvokedCallback="true"
         android:theme="@android:style/Theme.Material.Light.NoActionBar">
         <activity
             android:name=".MainActivity"