feat(21-01): add MAX_MESSAGES limit to sessionChatSlice.js

- Add MAX_MESSAGES = 500 constant to bound chat message storage
- Apply slice(-MAX_MESSAGES) in addMessageFromWebSocket reducer
- Apply slice(-MAX_MESSAGES) in fetchChatHistory.fulfilled reducer
- Apply slice(-MAX_MESSAGES) in uploadAttachment.fulfilled reducer

Fixes CHAT-02: Prevents unbounded memory growth from chat activity

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

@@ -2,6 +2,12 @@ import { createSlice, createAsyncThunk, createSelector } from '@reduxjs/toolkit'
 import { getChatMessages, sendChatMessage, uploadMusicNotation } from '../../helpers/rest';
 import { saveLastReadAt, loadLastReadAt } from '../../helpers/chatStorage';
 
+/**
+ * Maximum messages retained per channel to prevent unbounded memory growth
+ * Oldest messages are removed when this limit is exceeded
+ */
+const MAX_MESSAGES = 500;
+
 /**
  * Async thunk to fetch chat history for a channel
  * @param {Object} params - Request parameters
@@ -206,6 +212,9 @@ const sessionChatSlice = createSlice({
         new Date(a.createdAt) - new Date(b.createdAt)
       );
 
+      // Limit messages to MAX_MESSAGES to prevent unbounded memory growth
+      state.messagesByChannel[channel] = state.messagesByChannel[channel].slice(-MAX_MESSAGES);
+
       // Increment unread count if window closed OR viewing different channel
       if (!state.isWindowOpen || state.activeChannel !== channel) {
         state.unreadCounts[channel] = (state.unreadCounts[channel] || 0) + 1;
@@ -370,6 +379,9 @@ const sessionChatSlice = createSlice({
           new Date(a.createdAt) - new Date(b.createdAt)
         );
 
+        // Limit messages to MAX_MESSAGES to prevent unbounded memory growth
+        state.messagesByChannel[channel] = state.messagesByChannel[channel].slice(-MAX_MESSAGES);
+
         state.fetchStatus[channel] = 'succeeded';
         state.nextCursors[channel] = next;
 
@@ -515,6 +527,9 @@ const sessionChatSlice = createSlice({
             state.messagesByChannel[channel].sort((a, b) =>
               new Date(a.createdAt) - new Date(b.createdAt)
             );
+
+            // Limit messages to MAX_MESSAGES to prevent unbounded memory growth
+            state.messagesByChannel[channel] = state.messagesByChannel[channel].slice(-MAX_MESSAGES);
           }
         }
       })