feat(postit): wire Desktop address book to /api/user-search

Replaces the empty ContactService.Desktop stub with a real
implementation backed by UserSearchClient. Closes the loop
between the server-side /api/user-search endpoint (b3056f1c),
the client wrapper (6e7e0414), and the platform abstraction.

IContactService gains:
- SearchAsync(string query, CancellationToken): on desktop,
  hits /api/user-search and appends results to an in-memory
  cache. On mobile, throws PlatformNotSupportedException —
  mobile providers use the device-local address book
  (GetDeviceContactsAsync) and don't talk to a network search.
- Contacts (ObservableCollection<ContactDto>): live view of
  the cache; UI binds directly to it. Mobile populates it
  inside GetDeviceContactsAsync (eager load); desktop populates
  it via SearchAsync (lazy, on-demand).

ContactDto shape changes:
- Emails (IReadOnlyList<string>) -> Email (string?). The
  /api/user-search endpoint returns one email per user. The
  use case ('invite / add to a circle') only needs one.
- Mobile provider flattens its per-contact email list down
  to the first non-empty entry (a small functional loss that
  matches the wire shape).

App.axaml.cs constructs a ContactService from the
UserSearchClient singleton and registers it as
IContactService so future ViewModels can take the interface
by constructor injection.

Build + 51/51 tests green. The mobile provider is still
gated by #if ANDROID || IOS and not exercised by the
Desktop test target — runtime behaviour on Android will
need a smoke test on device when PostIt.Android lands.

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

@@ -1,6 +1,7 @@
 #if ANDROID || IOS
 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Maui.ApplicationModel.Communication;
@@ -24,6 +25,8 @@ namespace PostIt.Services;
 /// </summary>
 public sealed class ContactService : IContactService
 {
+    public ObservableCollection<ContactDto> Contacts { get; } = new();
+
     public async Task<IReadOnlyList<ContactDto>> GetDeviceContactsAsync(CancellationToken ct = default)
     {
         if (DeviceInfo.Current.Platform == DevicePlatform.Unknown)
@@ -38,21 +41,18 @@ public sealed class ContactService : IContactService
             var contacts = await Contacts.Default.GetAllAsync();
             if (contacts is null) return Array.Empty<ContactDto>();
 
-            var result = new List<ContactDto>();
+            // Flatten the per-contact email list down to one
+            // primary email. The platform-neutral ContactDto only
+            // carries one; the use case ("invite / add to a
+            // circle") only needs one. The first non-empty entry
+            // wins.
+            Contacts.Clear();
             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));
+                var email = FlattenPrimaryEmail(c.Emails);
+                Contacts.Add(new ContactDto(c.Id, c.DisplayName ?? string.Empty, email));
             }
-            return result;
+            return Contacts.ToArray();
         }
         catch (Exception ex)
         {
@@ -60,5 +60,22 @@ public sealed class ContactService : IContactService
             return Array.Empty<ContactDto>();
         }
     }
+
+    public Task SearchAsync(string query, CancellationToken ct = default)
+        => throw new PlatformNotSupportedException(
+            "SearchAsync is not supported on mobile — use GetDeviceContactsAsync " +
+            "to load the local address book. The network search lives on the " +
+            "desktop service, which queries the central user-search endpoint.");
+
+    private static string? FlattenPrimaryEmail(IEnumerable<EmailAddress>? emails)
+    {
+        if (emails is null) return null;
+        foreach (var e in emails)
+        {
+            if (!string.IsNullOrEmpty(e.EmailAddress))
+                return e.EmailAddress;
+        }
+        return null;
+    }
 }
-#endif
+#endif