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>

src/PostIt/PostIt/PostIt.csproj

@@ -3,11 +3,13 @@
     <TargetFramework>net10.0</TargetFramework>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
+     <UseMaui>true</UseMaui>
     <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
     <AssemblyVersion>1.0.1.0</AssemblyVersion>
     <FileVersion>1.0.1.0</FileVersion>
     <InformationalVersion>1.0.1-5+Branch.main.Sha.0617fc6bda7151c70559d87177e2dcfb1b60995f</InformationalVersion>
     <Version>1.0.1-5</Version>
+
   </PropertyGroup>
   <ItemGroup>
     <AvaloniaResource Include="Assets\**" />
@@ -24,6 +26,7 @@
     <PackageReference Include="CommunityToolkit.Mvvm" />
     <PackageReference Include="IdentityModel.OidcClient" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" />
+    <PackageReference Include="Microsoft.Maui.Essentials" />
     <ProjectReference Include="../../Yavsc.Abstract/Yavsc.Abstract.csproj" />
     <ProjectReference Include="../../Yavsc.Api.Client/Yavsc.Api.Client.csproj" />
   </ItemGroup>

src/PostIt/PostIt/Services/ContactService.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Maui.ApplicationModel.Communication;
+using Microsoft.Maui.ApplicationModel;
+using Microsoft.Maui.Devices;
+
+public class ContactService
+{
+    public async Task<IEnumerable<Contact>> GetDeviceContactsAsync()
+    {
+        // 1. Ensure the platform supports MAUI Essentials APIs
+        if (DeviceInfo.Current.Platform == DevicePlatform.Unknown)
+        {
+            throw new PlatformNotSupportedException("MAUI Essentials is not available on this platform.");
+        }
+
+        try
+        {
+            // 2. Request runtime permission (Required for Android & iOS)
+            var status = await Permissions.RequestAsync<Permissions.ContactsRead>();
+            if (status != PermissionStatus.Granted)
+            {
+                // Permission denied by user
+                return Array.Empty<Contact>();
+            }
+
+            // 3. Fetch all contacts
+            var contactsEnumerable = await Contacts.Default.GetAllAsync();
+            return contactsEnumerable ?? Array.Empty<Contact>();
+        }
+        catch (Exception ex)
+        {
+            // Handle cross-platform exceptions or logs here
+            System.Diagnostics.Debug.WriteLine($"Error fetching contacts: {ex.Message}");
+            return Array.Empty<Contact>();
+        }
+    }
+}