refactor: unify NOTE_WIDTH constant with --note-width CSS variable

Read note width from the CSS custom property at runtime (cached on first
call) instead of duplicating the value in a JS constant.

src/constants/note-width.ts

@@ -1 +1,12 @@
-export const NOTE_WIDTH = 480
+let cached: number | undefined
+
+export const getNoteWidth = () => {
+  if (cached === undefined) {
+    cached = parseInt(
+      getComputedStyle(document.documentElement).getPropertyValue(
+        "--note-width",
+      ),
+    )
+  }
+  return cached
+}