refactor(arch): split App into shared library + Desktop + Android heads

The Avalonia UI code (App, Views, ViewModels, Localization, Assets) now
lives in src/App as a plain library (no OutputType). Two thin heads
reference it:

- src/App.Desktop/ — WinExe, Avalonia.Desktop, hosts MainView in a
  MainWindow; carries Program.cs and app.manifest (moved from src/App)
- src/App.Android/ — net10.0-android, Avalonia.Android, MainActivity/
  MainApplication; targets API 23+, EmbedAssembliesIntoApk=true so the
  apk is self-contained for sideloading

All event handlers and TopLevel-dependent calls (file picker, clipboard,
folder picker) moved from MainWindow.axaml.cs into the new shared
MainView.axaml.cs (UserControl), using TopLevel.GetTopLevel(this) so
they work on both platforms. Esc/Back key handling is also in MainView.
MainWindow becomes a thin shell that hosts MainView.

Framework bump: all projects move to net10.0; Cli and Tests follow.

src/App/App.axaml.cs

@@ -1,31 +1,34 @@
-using Avalonia;
-using Avalonia.Controls.ApplicationLifetimes;
-using Avalonia.Data.Core;
-using Avalonia.Data.Core.Plugins;
-using System.Linq;
-using Avalonia.Markup.Xaml;
-using PalladiumWallet.App.ViewModels;
-using PalladiumWallet.App.Views;
-
-namespace PalladiumWallet.App;
-
-public partial class App : Application
-{
-    public override void Initialize()
-    {
-        AvaloniaXamlLoader.Load(this);
-    }
-
-    public override void OnFrameworkInitializationCompleted()
-    {
-        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
-        {
-            desktop.MainWindow = new MainWindow
-            {
-                DataContext = new MainWindowViewModel(),
-            };
-        }
-
-        base.OnFrameworkInitializationCompleted();
-    }
-}
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using PalladiumWallet.App.ViewModels;
+using PalladiumWallet.App.Views;
+
+namespace PalladiumWallet.App;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        var vm = new MainWindowViewModel();
+
+        // Desktop (Windows/Linux): finestra classica. Mobile (Android): vista
+        // singola. Stessa UI condivisa (MainView) e stesso ViewModel.
+        switch (ApplicationLifetime)
+        {
+            case IClassicDesktopStyleApplicationLifetime desktop:
+                desktop.MainWindow = new MainWindow { DataContext = vm };
+                break;
+            case ISingleViewApplicationLifetime singleView:
+                singleView.MainView = new MainView { DataContext = vm };
+                break;
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}