feat: periodic server ping every 5s while app is open

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/wzp/ui/call/InCallScreen.kt

@@ -90,10 +90,13 @@ fun InCallScreen(
 
     var showManageRelays by remember { mutableStateOf(false) }
 
-    // Ping servers on launch — engine init + QUIC ping, no restart needed
+    // Periodic ping every 5 seconds while app is open
     LaunchedEffect(Unit) {
         viewModel.loadSavedFingerprints()
+        while (true) {
             viewModel.pingAllServers()
+            kotlinx.coroutines.delay(5000)
+        }
     }
 
     Surface(