🎙️ a11y: Screen Reader Support for Dynamic Content Updates (#3625)

* WIP: first pass, hooks

* wip: isStream arg

* feat: first pass, dynamic content updates, screen reader announcements

* chore: unrelated, styling redundancy

client/src/a11y/MessageBlock.tsx

@@ -0,0 +1,26 @@
+import React from 'react';
+
+const offScreenStyle: React.CSSProperties = {
+  border: 0,
+  clip: 'rect(0 0 0 0)',
+  height: '1px',
+  margin: '-1px',
+  overflow: 'hidden',
+  whiteSpace: 'nowrap',
+  padding: 0,
+  width: '1px',
+  position: 'absolute',
+};
+
+interface MessageBlockProps {
+  message: string;
+  'aria-live': 'polite' | 'assertive';
+}
+
+const MessageBlock: React.FC<MessageBlockProps> = ({ message, 'aria-live': ariaLive }) => (
+  <div style={offScreenStyle} role="log" aria-live={ariaLive}>
+    {message}
+  </div>
+);
+
+export default MessageBlock;