WIP app invite: scaffold MAUI Essentials dependency in shared PostIt

Adds Microsoft.Maui.Essentials package and <UseMaui>true</UseMaui> to
src/PostIt/PostIt/PostIt.csproj so the shared project can compile code
that calls MAUI Essentials APIs (Microsoft.Maui.ApplicationModel.*).

Also adds a draft ContactService that wraps Contacts.Default.GetAllAsync()
behind a runtime platform check and permission request.

WIP caveats:
- The portable MAUI Essentials facade compiles on net10.0 but throws
  NotImplementedInReferenceAssemblyException at runtime when no
  platform-specific MAUI Essentials binary is loaded. A PostIt.Android
  project (or equivalent) must reference the Android MAUI Essentials
  implementation for Contacts.Default.GetAllAsync() to actually work.
- On desktop (Linux/macOS/Windows) the API is unsupported by design;
  ContactService currently throws PlatformNotSupportedException. A
  desktop stub returning Array.Empty<Contact>() is the likely next step.
- No tests yet. The scaffold is unverified at runtime; build passes.

src/PostIt/Directory.Packages.props

@@ -14,6 +14,7 @@
     <PackageVersion Include="CommunityToolkit.Mvvm" Version="8.4.2" />
     <PackageVersion Include="Material.Avalonia" Version="3.17.0" />
     <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="10.0.9" />
+    <PackageVersion Include="Microsoft.Maui.Essentials" Version="10.0.90" />
     <PackageVersion Include="Xamarin.AndroidX.Browser" Version="1.8.0" />
     <PackageVersion Include="Xamarin.AndroidX.Core.SplashScreen" Version="1.2.0" />
   </ItemGroup>