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

src/components/MainContent.tsx

@@ -1,4 +1,5 @@
 import React, { useState, useEffect, useRef, useCallback } from 'react';
+import './MainContent.css';
 import { createPortal } from 'react-dom';
 import { useProjectStore } from '../stores/projectStore';
 import { KGCore } from '../core/KGCore';