feat(demo): build a static editor preview for GitHub Pages

web/src/common/i18n/en.json

@@ -1,4 +1,5 @@
 {
+  "demo_notice": "Editor preview only. Changes are not saved; refreshing restores the sample.",
   "retry_b8784c": "Retry",
   "fonts_could_not_be_loaded_42a574": "Fonts could not be loaded. Check your connection and refresh the page.",
   "refresh_page_9366e1": "Refresh page",

web/src/common/i18n/zh-CN.json

@@ -1,4 +1,5 @@
 {
+  "demo_notice": "仅供编辑器预览。修改不会保存，刷新后恢复示例。",
   "retry_b8784c": "重试",
   "fonts_could_not_be_loaded_42a574": "字体加载失败，请检查网络后刷新页面。",
   "refresh_page_9366e1": "刷新页面",