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.Desktop/Program.cs

@@ -0,0 +1,24 @@
+using Avalonia;
+using System;
+
+namespace PalladiumWallet.App;
+
+sealed class Program
+{
+    // Initialization code. Don't use any Avalonia, third-party APIs or any
+    // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+    // yet and stuff might break.
+    [STAThread]
+    public static void Main(string[] args) => BuildAvaloniaApp()
+        .StartWithClassicDesktopLifetime(args);
+
+    // Avalonia configuration, don't remove; also used by visual designer.
+    public static AppBuilder BuildAvaloniaApp()
+        => AppBuilder.Configure<App>()
+            .UsePlatformDetect()
+#if DEBUG
+            .WithDeveloperTools()
+#endif
+            .WithInterFont()
+            .LogToTrace();
+}