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/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>