fix: prevent call screen from closing before state transitions

When initiateCall() runs async (Dispatchers.IO), navigation to
the ActiveCall screen happens before CallManager state changes
from Idle to Offering. The CallScreen sees Idle and immediately
calls onCallEnded() → popBack(), causing the screen to flash
and disappear.

Fix: CallScreen now waits 500ms on Idle before popping back.
If the state transitions to Offering within that window (normal
case), the screen stays. If it's still Idle after 500ms (no call
in progress), it pops back as before.

https://claude.ai/code/session_017hZm7yu7CzmcQgZGSaqSXS

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/call/CallScreen.kt

@@ -107,7 +107,14 @@ fun CallScreen(
     Box(modifier = Modifier.fillMaxSize()) {
         when (val state = callState) {
             is CallState.Idle -> {
-                LaunchedEffect(Unit) { onCallEnded() }
+                // Wait briefly — initiateCall runs async and state may not have
+                // transitioned yet when navigating to this screen
+                LaunchedEffect(Unit) {
+                    delay(500)
+                    if (callManager.state.value is CallState.Idle) {
+                        onCallEnded()
+                    }
+                }
             }
 
             is CallState.Offering -> {