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/MainApplication.cs

@@ -0,0 +1,32 @@
+using System;
+using Android.App;
+using Android.Runtime;
+using Avalonia;
+using Avalonia.Android;
+using PalladiumWallet.Core.Storage;
+
+namespace PalladiumWallet.Mobile;
+
+// In Avalonia 12 l'AppBuilder Android si configura nella sottoclasse Application
+// (AvaloniaAndroidApplication<TApp>), non più nell'Activity. allowBackup=false:
+// il file wallet cifrato/seed non deve finire nei backup cloud automatici.
+[Application(Label = "Palladium Wallet", AllowBackup = false)]
+public class MainApplication : AvaloniaAndroidApplication<global::PalladiumWallet.App.App>
+{
+    public MainApplication(IntPtr javaReference, JniHandleOwnership transfer)
+        : base(javaReference, transfer)
+    {
+    }
+
+    public override void OnCreate()
+    {
+        // Storage sandbox dell'app: wallet, configurazione e certificati vivono
+        // qui. Impostato prima dell'init Avalonia (che crea il ViewModel e decide
+        // se mostrare lo step "scegli cartella dati" del wizard).
+        AppPaths.OverrideDataRoot = FilesDir?.AbsolutePath;
+        base.OnCreate();
+    }
+
+    protected override AppBuilder CustomizeAppBuilder(AppBuilder builder) =>
+        base.CustomizeAppBuilder(builder).WithInterFont();
+}