style(FN-1847): unify dialog overlay styling and add accessibility attributes

- Add role=dialog and aria-modal attributes to all modal dialogs for accessibility
- Standardize close button aria-labels across all modals
- Unify overlay backdrop-filter, z-index, and background styles in styles.css
- Add light theme overrides for non-standard overlay backgrounds
- Update AgentListModal tests to reflect new aria attributes
- Add changeset for @gsxdsm/fusion package

packages/dashboard/app/components/ChatView.tsx

@@ -75,7 +75,7 @@ function NewChatDialog({ onClose, onCreate }: NewChatDialogProps) {
   };
 
   return (
-    <div className="chat-new-dialog-backdrop" onClick={onClose}>
+    <div className="chat-new-dialog-backdrop" onClick={onClose} role="dialog" aria-modal="true">
       <div className="chat-new-dialog" onClick={(e) => e.stopPropagation()}>
         <h3>New Chat</h3>
         <form onSubmit={handleSubmit}>