Update auth, JSON config, and sender dashboard styles

Modified the `[Authorize]` attribute in `EnvelopeController` to use `AuthScheme.Sender` for authentication. Updated `Program.cs` to configure JSON serialization with `ReferenceHandler.IgnoreCycles` to handle circular references.

Added new CSS styles for the sender dashboard in `sender-page.css`, including layout, action bars, buttons, tabs, badges, and responsive design improvements. Enhanced button states and introduced styles for status indicators and receiver badges.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -54,7 +54,11 @@ try
         .AddInteractiveWebAssemblyComponents();
 
     // Add API Controllers
-    builder.Services.AddControllers();
+    builder.Services.AddControllers()
+    .AddJsonOptions(options =>
+    {
+        options.JsonSerializerOptions.ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.IgnoreCycles;
+    });
     builder.Services.AddHttpClient();
 
     // YARP Reverse Proxy (for forwarding auth requests to AuthHub)