fix(ai-assistant): 修复图片预览调用不存在方法的错误

将 $A.previewFile 替换为 this.$store.dispatch("previewImage")，
解决 TypeError: $A.previewFile is not a function 错误。

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

resources/assets/js/components/AIAssistant/prompt-image.vue

@@ -42,11 +42,7 @@ export default {
         },
         showPreview() {
             if (this.imageUrl) {
-                $A.previewFile({
+                this.$store.dispatch("previewImage", this.imageUrl);
-                    type: 'image',
-                    url: this.imageUrl,
-                    name: `image_${this.imageId}`,
-                });
             }
         },
     },