refactor(postit): split IContactService from IUserDirectory

IContactService used to be the catch-all for "people you can reach
from PostIt": on mobile it read the device-local address book, on
desktop it queried the central /api/user-search endpoint and merged
both worlds into a single ContactDto (a flat Email field, an
ObservableCollection cache, a SearchAsync method). Two unrelated
flows under the same name, with a wire shape (Email) silently
flattening the mobile provider's multi-email list.

Split into two interfaces, each with a single responsibility:

- IContactService: device-local address book only. Mobile provider
  reads MAUI Essentials Contacts.Default and carries the full email
  list per contact. Desktop provider is an honest stub returning an
  empty list — the desktop has no local address book, and inviting
  external people from desktop is a separate flow (manual email
  entry + invitation endpoint) that doesn't belong here.

- IUserDirectory: central Yavsc user directory, the only consumer
  of /api/user-search. Both Desktop and Mobile providers delegate
  to UserSearchClient; the platform split exists so future
  platform-specific sources (offline cache, directory-scoped
  providers) can plug in without disturbing consumers.

ContactDto restores IReadOnlyList<string> Emails (the flat Email
from d0e0f4c1 was a regression that matched the wire shape of
/api/user-search at the cost of the mobile provider's per-contact
list). UserSummary is a separate platform-neutral record that
mirrors the server's UserSearchResultDto without leaking transport
concerns.

App.axaml.cs registers both interfaces as singletons.

Build + 51/51 PostIt.Tests green. No UI consumer yet — these
interfaces are still plomberie; the ViewModel that joins them for
the "add to a circle" / "invite someone" flows is a follow-up.

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

@@ -1,7 +1,6 @@
 #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;
@@ -11,22 +10,23 @@ using Microsoft.Maui.Devices;
 namespace PostIt.Services;
 
 /// <summary>
-/// Mobile implementation backed by MAUI Essentials Contacts.Default.
+/// Mobile implementation backed by MAUI Essentials
+/// <c>Contacts.Default</c>.
 ///
-/// Compiled only for ANDROID and IOS. On desktop targets, see
-/// ContactService.Desktop.cs (the stub that wins at compile time).
+/// <para>Compiled only for ANDROID and IOS. On desktop targets,
+/// see <c>ContactService.Desktop.cs</c> (the stub that wins at
+/// compile time).</para>
 ///
-/// 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.
+/// <para>Note: at runtime, this class throws
+/// <c>NotImplementedInReferenceAssemblyException</c> unless
+/// the host application project also references the
+/// platform-specific Microsoft.Maui.Essentials implementation
+/// (typically <c>PostIt.Android</c>). On iOS the same is
+/// required via <c>PostIt.iOS</c>. On desktop the stub is used
+/// and this file is excluded.</para>
 /// </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)
@@ -41,18 +41,24 @@ public sealed class ContactService : IContactService
             var contacts = await Contacts.Default.GetAllAsync();
             if (contacts is null) return Array.Empty<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();
+            // Carry the per-contact email list as-is. A real
+            // device contact can carry several addresses (home /
+            // work / other); the UI use case ("invite / add to a
+            // circle") can then decide which address to use, or
+            // let the user pick. The platform-neutral ContactDto
+            // shape is intentionally richer than the Yavsc
+            // directory's single-Email shape — the two flows
+            // answer different questions.
+            var result = new List<ContactDto>(contacts.Count);
             foreach (var c in contacts)
             {
-                var email = FlattenPrimaryEmail(c.Emails);
-                Contacts.Add(new ContactDto(c.Id, c.DisplayName ?? string.Empty, email));
+                var emails = ExtractEmails(c.Emails);
+                result.Add(new ContactDto(
+                    c.Id,
+                    c.DisplayName ?? string.Empty,
+                    emails));
             }
-            return Contacts.ToArray();
+            return result;
         }
         catch (Exception ex)
         {
@@ -61,21 +67,16 @@ public sealed class ContactService : IContactService
         }
     }
 
-    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)
+    private static IReadOnlyList<string> ExtractEmails(IEnumerable<EmailAddress>? emails)
     {
-        if (emails is null) return null;
+        if (emails is null) return Array.Empty<string>();
+        var list = new List<string>();
         foreach (var e in emails)
         {
             if (!string.IsNullOrEmpty(e.EmailAddress))
-                return e.EmailAddress;
+                list.Add(e.EmailAddress);
         }
-        return null;
+        return list;
     }
 }
-#endif
+#endif