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/App.axaml.cs

@@ -8,6 +8,10 @@ namespace PalladiumWallet.App;
 
 public partial class App : Application
 {
+    /// <summary>Set once the single ViewModel is created; lets platform heads (e.g. the
+    /// Android activity) reach it for lifecycle events without a second instance.</summary>
+    public static MainWindowViewModel? MainViewModel { get; private set; }
+
     public override void Initialize()
     {
         AvaloniaXamlLoader.Load(this);
@@ -16,6 +20,7 @@ public partial class App : Application
     public override void OnFrameworkInitializationCompleted()
     {
         var vm = new MainWindowViewModel();
+        MainViewModel = vm;
 
         // Desktop (Windows/Linux): classic window. Mobile (Android): single
         // view. Same shared UI (MainView) and same ViewModel.