feat(08-02): create JKChatMessage component

Create individual message display component with:
- Avatar with initials (first + last name initial)
- Sender name in bold
- Relative timestamp using formatTimestamp utility
- Message text with word wrapping
- Inline styles for MVP (SCSS styling deferred to Plan 8.3)
- React.memo for performance optimization
- PropTypes validation

Component follows established React patterns with functional components,
hooks, and proper documentation.

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

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

@@ -0,0 +1,87 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+import { formatTimestamp } from '../../../utils/formatTimestamp';
+
+/**
+ * Get initials for avatar from sender name
+ * Returns first letter of first and last name, or just first letter if single name
+ * @param {string} name - Sender name
+ * @returns {string} Initials (e.g., "JD" for "John Doe")
+ */
+const getInitials = (name) => {
+  if (!name) return '?';
+  const parts = name.trim().split(' ');
+  if (parts.length === 1) return parts[0][0].toUpperCase();
+  return (parts[0][0] + parts[parts.length - 1][0]).toUpperCase();
+};
+
+/**
+ * JKChatMessage - Individual message display component
+ *
+ * Displays a single chat message with avatar, sender name, message text, and timestamp.
+ *
+ * Features:
+ * - Avatar with initials (first + last name)
+ * - Sender name in bold
+ * - Relative timestamp (via formatTimestamp utility)
+ * - Message text with word wrapping
+ * - React.memo for performance optimization
+ */
+const JKChatMessage = ({ message }) => {
+  const { senderName, message: text, createdAt } = message;
+
+  return (
+    <div style={{
+      display: 'flex',
+      gap: '12px',
+      marginBottom: '16px',
+      padding: '8px',
+      borderRadius: '4px',
+      backgroundColor: '#f8f9fa'
+    }}>
+      {/* Avatar */}
+      <div style={{
+        width: '36px',
+        height: '36px',
+        borderRadius: '50%',
+        backgroundColor: '#007bff',
+        color: 'white',
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        fontWeight: 'bold',
+        fontSize: '14px',
+        flexShrink: 0
+      }}>
+        {getInitials(senderName)}
+      </div>
+
+      {/* Message content */}
+      <div style={{ flex: 1, minWidth: 0 }}>
+        <div style={{ display: 'flex', alignItems: 'baseline', gap: '8px', marginBottom: '4px' }}>
+          <span style={{ fontWeight: 600, fontSize: '14px', color: '#212529' }}>
+            {senderName || 'Unknown'}
+          </span>
+          <span style={{ fontSize: '12px', color: '#6c757d' }}>
+            {formatTimestamp(createdAt)}
+          </span>
+        </div>
+        <div style={{ fontSize: '14px', color: '#212529', wordWrap: 'break-word' }}>
+          {text}
+        </div>
+      </div>
+    </div>
+  );
+};
+
+JKChatMessage.propTypes = {
+  message: PropTypes.shape({
+    id: PropTypes.string.isRequired,
+    senderId: PropTypes.string.isRequired,
+    senderName: PropTypes.string,
+    message: PropTypes.string.isRequired,
+    createdAt: PropTypes.string.isRequired
+  }).isRequired
+};
+
+export default React.memo(JKChatMessage);