feat: add readonly-chat class to readonly embed (#4714)

Adds a 'readonly' prop to ChatContainer that conditionally applies a
'readonly-chat' CSS class to the chat container element. This allows
streamers using OBS to target the readonly chat embed with custom CSS.

The readonly embed at /embed/chat/readonly now passes readonly={true}
to ChatContainer, which adds the class to the container div.

Fixes #4266

Co-authored-by: Gabe Kangas <gabek@real-ity.com>

web/pages/embed/chat/readonly/index.tsx

@@ -35,6 +35,7 @@ export default function ReadOnlyChatEmbed() {
             showInput={false}
             height="100vh"
             chatAvailable={isChatAvailable}
+            readonly
           />
         )}
       </ErrorBoundary>