feat(07-01): implement sessionChatSlice with initial state structure

GREEN phase of TDD:
- Create sessionChatSlice.js with complete initial state
- All 12 state fields match CHAT_REDUX_DESIGN.md specification
- Multi-channel architecture: messagesByChannel keyed by channel ID
- Unread tracking: unreadCounts, lastReadAt per channel
- State machines: fetchStatus/error per channel, sendStatus/error global
- Pagination support: nextCursors per channel
- UI state: isWindowOpen, windowPosition
- Register slice in store.js as sessionChat reducer

All 13 tests pass. Ready for reducers in Task 2.

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

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

@@ -0,0 +1,43 @@
+import { createSlice } from '@reduxjs/toolkit';
+
+/**
+ * Initial state for session chat
+ * @type {Object}
+ * @property {Object.<string, Array>} messagesByChannel - Messages organized by channel ID
+ * @property {string|null} activeChannel - Currently active channel ID
+ * @property {string|null} channelType - Type of active channel ('global', 'session', 'lesson')
+ * @property {Object.<string, number>} unreadCounts - Unread message count per channel
+ * @property {Object.<string, string|null>} lastReadAt - ISO timestamp of last read per channel
+ * @property {Object.<string, string>} fetchStatus - Fetch status per channel ('idle', 'loading', 'succeeded', 'failed')
+ * @property {Object.<string, string|null>} fetchError - Fetch error message per channel
+ * @property {string} sendStatus - Send status ('idle', 'loading', 'succeeded', 'failed')
+ * @property {string|null} sendError - Send error message
+ * @property {Object.<string, number|null>} nextCursors - Pagination cursors per channel
+ * @property {boolean} isWindowOpen - Whether chat window is open
+ * @property {Object|null} windowPosition - Window position {x, y}
+ */
+const initialState = {
+  messagesByChannel: {},
+  activeChannel: null,
+  channelType: null,
+  unreadCounts: {},
+  lastReadAt: {},
+  fetchStatus: {},
+  fetchError: {},
+  sendStatus: 'idle',
+  sendError: null,
+  nextCursors: {},
+  isWindowOpen: false,
+  windowPosition: null
+};
+
+const sessionChatSlice = createSlice({
+  name: 'sessionChat',
+  initialState,
+  reducers: {
+    // Reducers will be added in Tasks 2-3
+  }
+});
+
+export const {} = sessionChatSlice.actions;
+export default sessionChatSlice.reducer;

jam-ui/src/store/store.js

@@ -15,6 +15,7 @@ import activeSessionReducer from "./features/activeSessionSlice"
 import sessionUIReducer from "./features/sessionUISlice"
 import mixersReducer from "./features/mixersSlice"
 import mediaReducer from "./features/mediaSlice"
+import sessionChatReducer from "./features/sessionChatSlice"
 
 export default configureStore({
   reducer: {
@@ -25,6 +26,7 @@ export default configureStore({
     session: sessionReducer, // this is the slice that holds the session lists
     activeSession: activeSessionReducer, // this is the slice that holds the currently active session
     sessionUI: sessionUIReducer, // this is the slice that holds the session UI state (modals, panels)
+    sessionChat: sessionChatReducer, // Phase 7: session chat state (messages, channels, unread tracking)
     mixers: mixersReducer, // Phase 5: mixer state (chat, broadcast, track mixers, metronome, media summary)
     media: mediaReducer, // Phase 5: media data (backing tracks, jam tracks, recorded tracks)
     latency: latencyReducer,