style(ui): align admin styling with ScadaLink master conventions

- Move CSS into wwwroot/css/ (theme.css, site.css); sidebar 218 -> 220px
- Add hamburger + Bootstrap collapse for <lg viewports
- Add Components/Shared/ with LoadingSpinner, ToastNotification, StatusBadge
- Replace .page-title with flex + <h4 class="mb-0"> across 20 pages
- Convert NewCluster + IdentificationFields forms to card + h6 subsection pattern

src/Server/ZB.MOM.WW.OtOpcUa.Admin/Components/_Imports.razor

@@ -12,4 +12,5 @@
 @using ZB.MOM.WW.OtOpcUa.Admin.Components.Layout
 @using ZB.MOM.WW.OtOpcUa.Admin.Components.Pages
 @using ZB.MOM.WW.OtOpcUa.Admin.Components.Pages.Clusters
+@using ZB.MOM.WW.OtOpcUa.Admin.Components.Shared
 @using ZB.MOM.WW.OtOpcUa.Admin.Services