fix(sync): recover connection after Android screen lock/unlock

TcpClient.Connected only reflects the last known socket state, so a
connection killed silently while the phone was locked (Doze, mobile
radio suspend, NAT timeout) still reported IsConnected == true. The
keep-alive ping's failure was swallowed by an empty catch, so the
stale "connected" state never cleared and sync kept retrying on a
dead socket instead of reconnecting.

Treat a failed keep-alive ping as a disconnect (tear down the client
and reconnect), and add OnPause/OnResume to the Android activity to
force an immediate health check on resume instead of waiting for the
20s timer, which may itself be suspended during Doze.

src/App.Android/MainActivity.cs

@@ -4,6 +4,7 @@ using Android.Content;
 using Android.Content.PM;
 using Android.OS;
 using Avalonia.Android;
+using AvaloniaApp = PalladiumWallet.App.App;
 
 namespace PalladiumWallet.Mobile;
 
@@ -17,6 +18,7 @@ public class MainActivity : AvaloniaMainActivity
     internal const int ScanRequestCode = 9001;
     internal static TaskCompletionSource<string?>? ScanTcs;
     internal static MainActivity? Current;
+    private bool _wasPaused;
 
     protected override void OnCreate(Bundle? savedInstanceState)
     {
@@ -24,6 +26,26 @@ public class MainActivity : AvaloniaMainActivity
         Current = this;
     }
 
+    protected override void OnPause()
+    {
+        base.OnPause();
+        _wasPaused = true;
+    }
+
+    protected override void OnResume()
+    {
+        base.OnResume();
+        if (!_wasPaused) return;
+        _wasPaused = false;
+        // The TCP socket can die silently while the screen was off/locked (Doze,
+        // mobile radio suspend, NAT timeout) without the app ever observing the
+        // failure. Force an immediate health check instead of waiting for the next
+        // 20s keep-alive tick, which may itself have been suspended for longer than
+        // the lock.
+        if (AvaloniaApp.MainViewModel is { } vm)
+            _ = vm.CheckConnectionOnResumeAsync();
+    }
+
     protected override void OnActivityResult(int requestCode, Result resultCode, Intent? data)
     {
         base.OnActivityResult(requestCode, resultCode, data);