ダークモードの同期を強化

src/client/scripts/is-device-darkmode.ts

@@ -1,3 +1,3 @@
 export function isDeviceDarkmode() {
-	return window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
+	return window.matchMedia('(prefers-color-scheme: dark)').matches;
 }