fix(phone-image): 修复字号倍数影响编辑器+刷新后丢失

- CSS变量 --pi-font-scale 仅在渲染容器上设置,不再全局污染
- Slider松手后立即保存配置,确保fontScale持久化

public/static/js/phone-image.js

@@ -80,7 +80,10 @@ var PhoneImageEngine = (function () {
      */
     function applyFontScale() {
         var scale = config.fontScale || 1;
-        document.documentElement.style.setProperty('--pi-font-scale', scale);
+        var preview = document.getElementById('render-preview');
+        if (preview) preview.style.setProperty('--pi-font-scale', scale);
+        var staging = document.getElementById('render-staging');
+        if (staging) staging.style.setProperty('--pi-font-scale', scale);
     }
 
     /**