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/PalladiumWallet.App.Desktop.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <!-- Head Desktop (Windows/Linux): entry-point e pacchetti specifici del
+       desktop. La UI vera vive nella libreria condivisa PalladiumWallet.App. -->
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ApplicationManifest>app.manifest</ApplicationManifest>
+    <ApplicationIcon>..\App\Assets\logo.ico</ApplicationIcon>
+    <AssemblyName>PalladiumWallet</AssemblyName>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" Version="12.0.4" />
+    <PackageReference Include="Avalonia.Desktop" 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>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\App\PalladiumWallet.App.csproj" />
+  </ItemGroup>
+</Project>