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.Android/MainActivity.cs

@@ -0,0 +1,16 @@
+using Android.App;
+using Android.Content.PM;
+using Avalonia.Android;
+
+namespace PalladiumWallet.Mobile;
+
+// Activity di avvio. La configurazione dell'app (tipo App, font) è nella
+// MainApplication (AvaloniaAndroidApplication<App>). Qui basta il launcher.
+[Activity(
+    Label = "Palladium Wallet",
+    Theme = "@style/MyTheme.NoActionBar",
+    MainLauncher = true,
+    ConfigurationChanges = ConfigChanges.Orientation | ConfigChanges.ScreenSize | ConfigChanges.UiMode)]
+public class MainActivity : AvaloniaMainActivity
+{
+}