feat: polished admin UI styles + email channels admin CRUD

- Extract inline styles to wwwroot/css/admin.css
- Bootstrap Icons for nav and buttons
- Styled page headers, table, empty state, readonly fields
- Email Channels admin: list, create, edit, delete
- GetAllEmailChannelsQuery + handler
- AdminChannelsController with full CRUD
- form id + form= attribute pattern for EditorLayout footer buttons

Ref: IT-628

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

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

@@ -3,7 +3,10 @@
 }
 <div class="card shadow-sm">
     <div class="card-header bg-primary text-white">
-        <h5 class="mb-0">@ViewData["EditorTitle"]</h5>
+        <h5 class="mb-0 editor-card-header">
+            <i class="bi bi-pencil-square"></i>
+            @ViewData["EditorTitle"]
+        </h5>
     </div>
     <div class="card-body">
         @RenderBody()