feat: add MVC Razor admin UI for email templates

- GetAllEmailTemplatesQuery + handler (new GetAll use case)
- IEmailTemplateRepository.GetAllAsync + EF implementation
- AdminTemplatesController: Index, Create, Edit, Save, Delete
- EmailTemplateEditViewModel with IsNew/PageTitle helpers
- Views/_ViewStart, _ViewImports, Shared/_Layout (Bootstrap 5)
- Shared/_EditorLayout (chained layout for all edit screens)
- Views/AdminTemplates/Index (table with edit/delete actions)
- Views/AdminTemplates/Edit (card form, readonly composite key on edit)
- Program.cs: AddControllersWithViews, UseStaticFiles, MapDefaultControllerRoute

Ref: IT-634

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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

@@ -0,0 +1,14 @@
+@{
+    Layout = "~/Views/Shared/_Layout.cshtml";
+}
+<div class="card shadow-sm">
+    <div class="card-header bg-primary text-white">
+        <h5 class="mb-0">@ViewData["EditorTitle"]</h5>
+    </div>
+    <div class="card-body">
+        @RenderBody()
+    </div>
+    <div class="card-footer d-flex gap-2">
+        @RenderSection("FormActions", required: false)
+    </div>
+</div>