feat(app-invite): isolate ContactService to mobile targets

Splits the single ContactService class (which threw
PlatformNotSupportedException on non-Android/iOS targets) into a
platform-conditional structure:

- IContactService + ContactDto: shared abstraction in
  src/PostIt/PostIt/Services/IContactService.cs. ViewModels depend
  on this; concrete providers map their native shapes to ContactDto.

- ContactService.Mobile.cs: MAUI Essentials implementation, compiled
  only when ANDROID or IOS is defined. Wraps
  Contacts.Default.GetAllAsync() with permission handling and a
  NotImplementedInReferenceAssemblyException safety net.

- ContactService.Desktop.cs: stub returning an empty list, compiled
  when neither ANDROID nor IOS is defined. Replaces the
  'throw PlatformNotSupportedException' path so desktop targets
  (PostIt.Desktop, PostIt.Browser) build and run cleanly.

The Microsoft.Maui.Essentials portable facade is referenced from
PostIt.csproj, but it only becomes functional when the host
application project (PostIt.Android, future PostIt.iOS) also
references the platform-specific implementation.

No tests added: per AGENTS.md, a 'stub returns empty list' test on
PostIt.Tests (net10.0 desktop target) would be cosmetic and not
detect the real failure mode. Android-side tests require a working
PostIt.Android project, which doesn't exist yet.

Future providers (Google Contacts API, Exchange, CardDAV) plug in
as additional IContactService implementations selected by DI
configuration.

src/PostIt/PostIt/Services/ContactService.Mobile.cs

@@ -0,0 +1,64 @@
+#if ANDROID || IOS
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Maui.ApplicationModel.Communication;
+using Microsoft.Maui.ApplicationModel;
+using Microsoft.Maui.Devices;
+
+namespace PostIt.Services;
+
+/// <summary>
+/// Mobile implementation backed by MAUI Essentials Contacts.Default.
+///
+/// Compiled only for ANDROID and IOS. On desktop targets, see
+/// ContactService.Desktop.cs (the stub that wins at compile time).
+///
+/// Note: at runtime, this class throws
+/// NotImplementedInReferenceAssemblyException unless the host
+/// application project also references the platform-specific
+/// Microsoft.Maui.Essentials implementation (typically the
+/// PostIt.Android project). On iOS the same is required via
+/// PostIt.iOS. On desktop the stub is used and this file is excluded.
+/// </summary>
+public sealed class ContactService : IContactService
+{
+    public async Task<IReadOnlyList<ContactDto>> GetDeviceContactsAsync(CancellationToken ct = default)
+    {
+        if (DeviceInfo.Current.Platform == DevicePlatform.Unknown)
+            return Array.Empty<ContactDto>();
+
+        try
+        {
+            var status = await Permissions.RequestAsync<Permissions.ContactsRead>();
+            if (status != PermissionStatus.Granted)
+                return Array.Empty<ContactDto>();
+
+            var contacts = await Contacts.Default.GetAllAsync();
+            if (contacts is null) return Array.Empty<ContactDto>();
+
+            var result = new List<ContactDto>();
+            foreach (var c in contacts)
+            {
+                var emails = new List<string>();
+                if (c.Emails is not null)
+                {
+                    foreach (var e in c.Emails)
+                    {
+                        if (!string.IsNullOrEmpty(e.EmailAddress))
+                            emails.Add(e.EmailAddress);
+                    }
+                }
+                result.Add(new ContactDto(c.Id, c.DisplayName ?? string.Empty, emails));
+            }
+            return result;
+        }
+        catch (Exception ex)
+        {
+            System.Diagnostics.Debug.WriteLine($"ContactService: {ex.Message}");
+            return Array.Empty<ContactDto>();
+        }
+    }
+}
+#endif