use svelte class directive for conditional classes

vite.config.ts

@@ -1,8 +1,14 @@
 import { sveltekit } from '@sveltejs/kit/vite';
 import { defineConfig } from 'vitest/config';
+import { multicssclass } from 'svelte-multicssclass';
 
 export default defineConfig({
-	plugins: [sveltekit()],
+	plugins: [
+		// eslint-disable-next-line @typescript-eslint/ban-ts-comment
+		// @ts-ignore
+		multicssclass(),
+		sveltekit()
+	],
 	test: {
 		include: ['src/**/*.{test,spec}.{ts,js}']
 	}