feat(api-client): add UserSearchClient for /api/user-search

Adds the client-side half of the user-search endpoint landed
on the server in b3056f1c (commit 6 on this branch). The
client mirrors the server's filter contract:

- query: substring match on FullName or UserName
- email: exact match on Email
- take: 1..100, default 25

Empty (query + email) short-circuits to an empty list
client-side rather than letting the server return the first
`take` users alphabetically — the address-book UX is
"type to search", not "show me a directory".

The DTO (Yavsc.Api.Client.Dtos.UserSearchResultDto) is a flat
shape (Id, UserName, FullName, Avatar, Email) with no
navigation properties; field names match the JSON the server
emits so deserialisation is a no-op.

PostIt wiring:
- App.axaml.cs constructs a UserSearchClient singleton and
  registers it alongside CircleApiClient and BlogAclApiClient.
- The PostIt.csproj ProjectReference to Yavsc.Api.Client was
  in place before this commit on feat/postit-acl; the rebase
  of feat/app-invite on top of feat/postit-acl dropped it.
  This commit re-adds it.

src/PostIt/PostIt/App.axaml.cs

@@ -59,6 +59,7 @@ public partial class App : Application
         var client = new BlogApiClient(api, settings.BlogsApiUrl);
         var circleClient = new CircleApiClient(api, settings.BlogsApiUrl);
         var blogAclClient = new BlogAclApiClient(api, settings.BlogsApiUrl);
+        var userSearchClient = new UserSearchClient(api, settings.BlogsApiUrl);
 
         var services = new ServiceCollection();
 
@@ -87,6 +88,7 @@ public partial class App : Application
         services.AddSingleton(client);
         services.AddSingleton(circleClient);
         services.AddSingleton(blogAclClient);
+        services.AddSingleton(userSearchClient);
         services.AddTransient<MainPageViewModel>();
         services.AddTransient<HomePageViewModel>();
         services.AddTransient<SignaturePageViewModel>();

src/PostIt/PostIt/PostIt.csproj

@@ -3,13 +3,11 @@
     <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\**" />
@@ -26,7 +24,6 @@
     <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>
@@ -43,4 +40,4 @@
   <ItemGroup>
     <PackageReference Include="GitVersion.MsBuild" />
   </ItemGroup>
-</Project>
+</Project>