refactor(11-01): polish edge cases and add PropTypes

Polish edge case handling and add missing PropTypes for textarea ref.

- JKChatComposer: Add PropTypes for textareaRef prop, add defaultProps
- JKChatEmptyState: Update message to match plan spec ("Start the conversation!")
- Verified edge cases: clearSendError works with retry, rapid actions handled, empty states work
- All edge cases from plan verified: disconnection, long messages, rapid toggling, multiple unread

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/components/client/chat/JKChatComposer.js

@@ -1,4 +1,5 @@
 import React, { useState, useCallback, useMemo } from 'react';
+import PropTypes from 'prop-types';
 import { useDispatch, useSelector } from 'react-redux';
 import { sendMessage, selectSendStatus, selectSendError, clearSendError } from '../../../store/features/sessionChatSlice';
 import { selectSessionId } from '../../../store/features/activeSessionSlice';
@@ -221,4 +222,12 @@ const JKChatComposer = ({ textareaRef }) => {
   );
 };
 
+JKChatComposer.propTypes = {
+  textareaRef: PropTypes.object
+};
+
+JKChatComposer.defaultProps = {
+  textareaRef: null
+};
+
 export default React.memo(JKChatComposer);

jam-ui/src/components/client/chat/JKChatEmptyState.js

@@ -22,7 +22,7 @@ const JKChatEmptyState = () => {
         No messages yet
       </p>
       <p style={{ fontSize: '14px', margin: 0 }}>
-        Be the first to send a message in this chat!
+        Start the conversation!
       </p>
     </div>
   );