feat(ui/auth): use a minimal layout for the login page

The login page previously rendered inside MainLayout, showing the full
nav sidebar and the authenticated-user footer. It now uses a bare
LoginLayout (no nav, no session-expiry watchdog, no dialog host) and
just renders its own centred card.

src/ScadaLink.CentralUI/Components/Pages/Login.razor

@@ -1,4 +1,5 @@
 @page "/login"
+@layout LoginLayout
 @using Microsoft.AspNetCore.Authorization
 @attribute [AllowAnonymous]