feat(07-01): implement unread tracking reducers and complete slice

GREEN phase of TDD for Task 3:
- Implement markAsRead: resets unread count to 0, updates lastReadAt timestamp
- Implement incrementUnreadCount: safely handles missing channels
- Implement setWindowPosition: stores UI position for WindowPortal
- Export all 7 action creators from slice
- Complete sessionChatSlice with full reducer suite

All 40 tests pass including 3 integration tests:
- Complete flow validates message receiving, channel switching, window state
- Multi-channel flow validates independent unread counts per channel
- Deduplication test validates WebSocket + REST double-message scenario

Ready for async thunks in Phase 7 Plan 2.

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

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

@@ -111,6 +111,33 @@ const sessionChatSlice = createSlice({
      */
     closeChatWindow: (state) => {
       state.isWindowOpen = false;
+    },
+
+    /**
+     * Mark channel as read
+     * Resets unread count and updates lastReadAt timestamp
+     */
+    markAsRead: (state, action) => {
+      const { channel } = action.payload;
+      state.unreadCounts[channel] = 0;
+      state.lastReadAt[channel] = new Date().toISOString();
+    },
+
+    /**
+     * Increment unread count for a channel
+     * Initializes count to 1 if channel doesn't exist
+     */
+    incrementUnreadCount: (state, action) => {
+      const { channel } = action.payload;
+      state.unreadCounts[channel] = (state.unreadCounts[channel] || 0) + 1;
+    },
+
+    /**
+     * Set window position for persistence
+     * Used by WindowPortal to save drag position
+     */
+    setWindowPosition: (state, action) => {
+      state.windowPosition = action.payload;
     }
   }
 });
@@ -119,7 +146,10 @@ export const {
   addMessageFromWebSocket,
   setActiveChannel,
   openChatWindow,
-  closeChatWindow
+  closeChatWindow,
+  markAsRead,
+  incrementUnreadCount,
+  setWindowPosition
 } = sessionChatSlice.actions;
 
 export default sessionChatSlice.reducer;