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
+{
+}

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();
+}

src/App.Android/PalladiumWallet.App.Android.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Head Android: entry-point e pacchetti specifici. La UI vera vive nella
+       libreria condivisa PalladiumWallet.App (stessa di desktop). -->
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0-android</TargetFramework>
+    <SupportedOSPlatformVersion>23</SupportedOSPlatformVersion>
+    <Nullable>enable</Nullable>
+    <ApplicationId>io.github.davide3011.palladiumwallet</ApplicationId>
+    <!-- ApplicationVersion = versionCode (intero), ApplicationDisplayVersion = versionName -->
+    <ApplicationVersion>1</ApplicationVersion>
+    <ApplicationDisplayVersion>0.9.0</ApplicationDisplayVersion>
+    <AndroidPackageFormat>apk</AndroidPackageFormat>
+    <!-- Includi le assembly .NET DENTRO l'apk: senza, in Debug si usa il Fast
+         Deployment (assembly spinte via adb da `dotnet run`) e un apk installato
+         a mano — sideload sul telefono o `adb install` — crasha all'avvio con
+         "monodroid: No assemblies found ... Exiting". Così l'apk è autosufficiente. -->
+    <EmbedAssembliesIntoApk>true</EmbedAssembliesIntoApk>
+    <!-- Default: ABI multiple (l'apk gira sia su emulatore x86_64 sia su telefoni arm64).
+         Per un apk più piccolo solo-telefono: aggiungi -p:AbiArm64Only=true alla build. -->
+    <RuntimeIdentifiers Condition="'$(AbiArm64Only)' == 'true'">android-arm64</RuntimeIdentifiers>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia.Android" Version="12.0.4" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\App\PalladiumWallet.App.csproj" />
+  </ItemGroup>
+</Project>

src/App.Android/Properties/AndroidManifest.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+	<!-- Necessario per parlare col server di indicizzazione (TCP/TLS).
+	     L'elemento <application> (label, allowBackup) è generato dall'attributo
+	     [Application] su MainApplication. usesCleartextTraffic resta al default
+	     (bloccato su Android 9+): il wallet preferisce comunque TLS. -->
+	<uses-permission android:name="android.permission.INTERNET" />
+</manifest>

src/App.Android/Resources/values/styles.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <!-- DEVE derivare da Theme.AppCompat: AvaloniaActivity estende AppCompatActivity,
+         che con un tema non-AppCompat lancia IllegalStateException all'avvio.
+         Avalonia disegna comunque l'intera UI (NoActionBar). -->
+    <style name="MyTheme.NoActionBar" parent="Theme.AppCompat.Light.NoActionBar" />
+</resources>