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

src/components/piano-roll/PianoRoll.tsx

@@ -1,4 +1,5 @@
 import React, { useRef, useEffect, useState, useCallback } from 'react';
+import './PianoRoll.css';
 import type { MouseEvent } from 'react';
 import { useProjectStore } from '../../stores/projectStore';
 import { FaGripLines } from 'react-icons/fa';