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

src/main.tsx

@@ -1,6 +1,7 @@
 import 'reflect-metadata';
 import { StrictMode } from 'react';
 import { createRoot } from 'react-dom/client';
+import './styles/variables.css';
 import './index.css';
 import App from './App.tsx';
 import { KGCore } from './core/KGCore';