fix: ping every 5min (was 5s), clean endpoint on failure, never block connect

- Ping interval: 5 minutes (was 5 seconds — too aggressive)
- Rust ping_relay: explicitly close endpoint + shutdown runtime on failure
- Connect button works regardless of ping status (never blocked)
- Ping failure doesn't corrupt engine state

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

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

@@ -90,12 +90,13 @@ fun InCallScreen(
 
     var showManageRelays by remember { mutableStateOf(false) }
 
-    // Periodic ping every 5 seconds while app is open
+    // Ping once on launch, then every 5 minutes
     LaunchedEffect(Unit) {
         viewModel.loadSavedFingerprints()
+        viewModel.pingAllServers()
         while (true) {
+            kotlinx.coroutines.delay(300_000) // 5 minutes
             viewModel.pingAllServers()
-            kotlinx.coroutines.delay(5000)
         }
     }