feat(09-02): integrate JKChatComposer into chat window

- Added JKChatComposer import and component
- Updated layout with flex container for proper spacing
- Header fixed at top, message list scrollable in middle, composer at bottom
- Complete three-section window structure (header/list/composer)

jam-ui/src/components/client/JKSessionChatWindow.js

@@ -3,6 +3,7 @@ import { useSelector, useDispatch } from 'react-redux';
 import WindowPortal from '../common/WindowPortal.js';
 import JKChatHeader from './chat/JKChatHeader.js';
 import JKChatMessageList from './chat/JKChatMessageList.js';
+import JKChatComposer from './chat/JKChatComposer.js';
 import {
   selectIsChatWindowOpen,
   selectActiveChannel,
@@ -21,9 +22,12 @@ import {
  * - closeChatWindow: Action to close window
  *
  * Component hierarchy:
- * WindowPortal → JKChatHeader + JKChatMessageList
+ * WindowPortal → JKChatHeader + JKChatMessageList + JKChatComposer
  *
- * Note: Message composition implementation deferred to Plan 9
+ * Layout:
+ * - Header: Fixed at top (~50px)
+ * - Message List: Scrollable, fills middle space
+ * - Composer: Fixed at bottom (~120px)
  */
 const JKSessionChatWindow = () => {
   const dispatch = useDispatch();
@@ -56,12 +60,15 @@ const JKSessionChatWindow = () => {
       onClose={handleClose}
       windowId="jamkazam-chat"
     >
-      <div style={{ display: 'flex', flexDirection: 'column', height: '100%' }}>
+      <div style={{ display: 'flex', flexDirection: 'column', height: '100%', width: '100%' }}>
         <JKChatHeader
           channelName={getChannelName()}
           onClose={handleClose}
         />
-        <JKChatMessageList />
+        <div style={{ flex: 1, overflowY: 'auto' }}>
+          <JKChatMessageList />
+        </div>
+        <JKChatComposer />
       </div>
     </WindowPortal>
   );