Replace gradient with uniform 80% black background

- Remove radial gradient to fix banding issues
- Use solid 80% black background (rgba(0,0,0,0.8))
- Apply to both ModelSelectionModal and McpServerModal
- Clean, consistent backdrop without visual artifacts

examples/chat-ui/src/components/McpServerModal.tsx

@@ -241,7 +241,7 @@ const McpServerModal: React.FC<McpServerModalProps> = ({
     <>
       <div 
         className={`fixed inset-0 z-50 flex items-center justify-center p-4 ${isOpen ? 'block' : 'hidden'}`}
-        style={{ background: 'radial-gradient(circle, rgba(0,0,0,0.8) 0%, rgba(0,0,0,0.7) 100%)' }}
+        style={{ backgroundColor: 'rgba(0,0,0,0.8)' }}
         onClick={onClose}
       >
         <div 

examples/chat-ui/src/components/ModelSelectionModal.tsx

@@ -88,7 +88,7 @@ const ModelSelectionModal: React.FC<ModelSelectionModalProps> = ({
     <>
       <div 
         className="fixed inset-0 z-50 flex items-center justify-center p-4"
-        style={{ background: 'radial-gradient(circle, rgba(0,0,0,0.8) 0%, rgba(0,0,0,0.7) 100%)' }}
+        style={{ backgroundColor: 'rgba(0,0,0,0.8)' }}
         onClick={onClose}
       >
         <div