fix(ui/auth): stop /login redirect loop when the session is expired

SessionExpiry renders inside MainLayout, which also wraps the login
page. For a user with a still-present auth cookie but an expired
expires_at claim, it redirected /login back to /login indefinitely.
It now skips the redirect when already on the login page.

src/ScadaLink.CentralUI/Components/Shared/SessionExpiry.razor

@@ -7,6 +7,11 @@
 
     protected override async Task OnInitializedAsync()
     {
+        // The login page uses the same layout, so this component renders there
+        // too. Redirecting /login → /login would loop ("too many redirects").
+        var path = Navigation.ToBaseRelativePath(Navigation.Uri);
+        if (path.StartsWith("login", StringComparison.OrdinalIgnoreCase)) return;
+
         var auth = await AuthStateProvider.GetAuthenticationStateAsync();
         if (auth.User.Identity?.IsAuthenticated != true) return;