feat: revamp demo

demo/composables/useDemoDark.ts

@@ -0,0 +1,10 @@
+import { useDark } from "@vueuse/core";
+
+export function useDemoDark() {
+  return useDark({
+    storageKey: "vue-echarts-demo-theme",
+    valueLight: "light",
+    valueDark: "dark",
+    disableTransition: false,
+  });
+}