feat(21-01): add clearAllMessages action and dispatch on session leave

- Add clearAllMessages reducer to sessionChatSlice.js
- Export clearAllMessages action
- Dispatch clearAllMessages in handleLeaveSubmit (explicit leave)
- Dispatch clearAllMessages in unmount cleanup useEffect

Fixes CHAT-03: Prevents stale chat data accumulation across sessions

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

@@ -55,7 +55,7 @@ import {
   selectBackingTrackData,
   selectJamTrackData
 } from '../../store/features/activeSessionSlice';
-import { addMessageFromWebSocket, uploadAttachment, selectIsUploading, selectUploadError, selectUploadFileName, selectUploadStatus, clearUploadError, fetchChatHistory } from '../../store/features/sessionChatSlice';
+import { addMessageFromWebSocket, uploadAttachment, selectIsUploading, selectUploadError, selectUploadFileName, selectUploadStatus, clearUploadError, fetchChatHistory, clearAllMessages } from '../../store/features/sessionChatSlice';
 import { validateFile } from '../../services/attachmentValidation';
 
 import { CLIENT_ROLE, RECORD_TYPE_AUDIO, RECORD_TYPE_BOTH } from '../../helpers/globals';
@@ -912,6 +912,9 @@ const JKSessionScreen = () => {
       // Then perform leave operations
       await sessionModel.handleLeaveSession();
 
+      // Clear Redux chat state to prevent stale data
+      dispatch(clearAllMessages());
+
       // Clear Redux session state
       dispatch(clearSession());
 
@@ -940,6 +943,9 @@ const JKSessionScreen = () => {
         resetMetronome();
       }
 
+      // Clear Redux chat state to prevent stale data
+      dispatch(clearAllMessages());
+
       // Clear Redux session state on unmount
       dispatch(clearSession());
     };

jam-ui/src/store/features/sessionChatSlice.js

@@ -311,6 +311,20 @@ const sessionChatSlice = createSlice({
     clearUploadError: (state) => {
       state.uploadState.error = null;
       state.uploadState.status = 'idle';
+    },
+
+    /**
+     * Clear all messages when leaving session
+     * Called from JKSessionScreen when user leaves to prevent stale data
+     */
+    clearAllMessages: (state) => {
+      state.messagesByChannel = {};
+      state.unreadCounts = {};
+      state.fetchStatus = {};
+      state.fetchError = {};
+      state.nextCursors = {};
+      // Preserve lastReadAt for persistence across sessions
+      // Preserve isWindowOpen and windowPosition for UX continuity
     }
   },
   extraReducers: (builder) => {
@@ -552,7 +566,8 @@ export const {
   setWindowPosition,
   clearSendError,
   setUploadStatus,
-  clearUploadError
+  clearUploadError,
+  clearAllMessages
 } = sessionChatSlice.actions;
 
 // Phase 7 Plan 3: Memoized selectors using createSelector from Redux Toolkit