fix(client-controller): single constructor with IHtmlLocalizer

The partial class ClientController had two constructors declared
across ClientController.cs and ClientController.Collections.cs.
ASP.NET Core DI failed to pick one at request time with:

  System.InvalidOperationException: Multiple constructors accepting
  all given argument types have been found in type
  'Yavsc.Controllers.ClientController'.

Move IHtmlLocalizer<ClientController> into the primary constructor
in ClientController.cs and drop the duplicate one in
ClientController.Collections.cs. The Collections partial now keeps
only its readonly field and action methods; the constructor and
field assignment are unified on the main file.

Also add the missing 'using Microsoft.AspNetCore.Mvc.Localization;'
to ClientController.cs so IHtmlLocalizer resolves.

src/Yavsc.Org/Controllers/Administration/ClientController.Collections.cs

@@ -27,12 +27,8 @@ public partial class ClientController
 
     readonly IHtmlLocalizer _localizer;
 
-    public ClientController(
-        IHtmlLocalizer<ClientController> localizer
-        )
-         {
-            _localizer = localizer;
-        }
+    // ClientController has a single constructor declared in
+    // ClientController.cs; this partial shares its fields.
 
 
     [HttpGet]