refactor: split monolithic App.css into feature-based co-located CSS files

src/components/ChatBox.tsx

@@ -1,4 +1,5 @@
 import React, { useState, useRef, useEffect, memo, useCallback } from 'react';
+import './ChatBox.css';
 import { FaPlus, FaBan, FaDownload } from 'react-icons/fa';
 import { UserMessage, AssistantMessage } from './chat';
 import { AgentCore } from '../agent/core/AgentCore';