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/PalladiumWallet.App.csproj

@@ -1,35 +1,30 @@
-<Project Sdk="Microsoft.NET.Sdk">
-  <PropertyGroup>
-    <OutputType>WinExe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
-    <!-- Versione dell'applicazione: unico punto da modificare. Compare nel
-         titolo della finestra ed è incisa nei binari pubblicati. -->
-    <Version>0.9.0</Version>
-    <Nullable>enable</Nullable>
-    <ApplicationManifest>app.manifest</ApplicationManifest>
-    <ApplicationIcon>Assets\logo.ico</ApplicationIcon>
-    <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
-  </PropertyGroup>
-
-  <ItemGroup>
-    <Folder Include="Models\" />
-    <AvaloniaResource Include="Assets\**" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <PackageReference Include="Avalonia" Version="12.0.4" />
-    <PackageReference Include="Avalonia.Desktop" Version="12.0.4" />
-    <PackageReference Include="Avalonia.Themes.Fluent" Version="12.0.4" />
-    <PackageReference Include="Avalonia.Fonts.Inter" Version="12.0.4" />
-    <PackageReference Include="AvaloniaUI.DiagnosticsSupport" Version="2.2.1">
-      <IncludeAssets Condition="'$(Configuration)' != 'Debug'">None</IncludeAssets>
-      <PrivateAssets Condition="'$(Configuration)' != 'Debug'">All</PrivateAssets>
-    </PackageReference>
-    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.1" />
-    <PackageReference Include="QRCoder" Version="1.8.0" />
-  </ItemGroup>
-
-  <ItemGroup>
-    <ProjectReference Include="..\Core\PalladiumWallet.Core.csproj" />
-  </ItemGroup>
-</Project>
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Libreria UI condivisa: il codice Avalonia (App, Views, ViewModels, Loc,
+       Assets) usato sia dall'head Desktop sia dall'head Android. Gli head
+       portano l'entry-point e i pacchetti specifici di piattaforma. -->
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <!-- Versione dell'applicazione: unico punto da modificare. Compare nel
+         titolo della finestra ed è incisa nei binari pubblicati. -->
+    <Version>0.9.0</Version>
+    <Nullable>enable</Nullable>
+    <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Folder Include="Models\" />
+    <AvaloniaResource Include="Assets\**" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" Version="12.0.4" />
+    <PackageReference Include="Avalonia.Themes.Fluent" Version="12.0.4" />
+    <PackageReference Include="Avalonia.Fonts.Inter" Version="12.0.4" />
+    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.1" />
+    <PackageReference Include="QRCoder" Version="1.8.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Core\PalladiumWallet.Core.csproj" />
+  </ItemGroup>
+</Project>