Force request host/scheme to App__Domain behind a proxy

The cookie login redirect and other absolute URLs are built from Request.Host;
behind a proxy that doesn't forward the Host header that's the internal IP:port,
so hitting the domain 302'd to the server IP. Rewrite scheme+host to App__Domain
on every request (after UseForwardedHeaders) so all generated URLs stay on the
public domain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Core/Config/AppDomainOptions.cs

@@ -26,4 +26,11 @@ public class AppDomainOptions
 
         return domain + "/" + path.TrimStart('/');
     }
+
+    /// <summary>
+    /// The configured domain as an absolute base <see cref="Uri"/> (scheme + host [+ port]), or
+    /// <c>null</c> when no domain is set or it can't be parsed.
+    /// </summary>
+    public Uri? GetBaseUri() =>
+        BuildUrl("/") is { } url && Uri.TryCreate(url, UriKind.Absolute, out var uri) ? uri : null;
 }