feat(push): add service-owned firebase channels and delivery

Add push contracts, provider delivery and channel diagnostics. Link email channels to services and document migration and validation.

Ref: IT-1074

HrynCo.NotificationService.Web/Views/Shared/_Layout.cshtml

@@ -2,6 +2,7 @@
     var currentController = ViewContext.RouteData.Values["controller"]?.ToString() ?? "";
     bool isServices = currentController.Equals("AdminServices", StringComparison.OrdinalIgnoreCase)
         || currentController.Equals("AdminTemplates", StringComparison.OrdinalIgnoreCase);
+    bool isPushChannels = currentController.Equals("AdminPushChannels", StringComparison.OrdinalIgnoreCase);
     bool isChannels  = currentController.Equals("AdminChannels",  StringComparison.OrdinalIgnoreCase);
 }
 <!DOCTYPE html>
@@ -34,6 +35,11 @@
                             <i class="bi bi-broadcast"></i> Email Channels
                         </a>
                     </li>
+                    <li class="nav-item">
+                        <a class="nav-link @(isPushChannels ? "active" : "")" href="/admin/push-channels">
+                            <i class="bi bi-phone"></i> Push Channels
+                        </a>
+                    </li>
                 </ul>
             </nav>