fix(#787): make the image-preview title actually derive from the original URL

Verified against the bundled viewer.min.js: the `title(image, imageData)` callback
receives the <img> element Viewer.js creates internally (view(): `var image =
document.createElement('img')`), NOT the original <div>. Viewer only forwards the
attributes listed in `inheritedAttributes` (default: crossOrigin, decoding, isMap,
loading, referrerPolicy, sizes, srcset, useMap) when building that <img>, and
`data-original-url` is not in that list. So the previous title function always read
null and the title stayed 'image' -- the fix was a no-op at runtime.

Fix: add `data-original-url` to Viewer's `inheritedAttributes` so the original URL is
forwarded from the thumbnail <img> to the title <img>, letting
`getImageNameFromOriginalUrl` return the real filename (and not the CORS-proxy URL).

Also updates PictureFilePreviewTests to assert `inheritedAttributes` is configured
with data-original-url, locking the regression.

server/src/test/java/cn/keking/PictureFilePreviewTests.java

@@ -14,7 +14,12 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
  *
  * 根因：v4.3.0 之后模板把图片元素从 <img> 改成 <div src>，而 HTMLDivElement 没有 DOM 的
  * src 属性，Viewer.js 用 image.src 取标题得到 undefined，导致标题为空。修复方式是在 Viewer 初始化时
- * 配置 title 函数，从 data-original-url 派生文件名（该属性在元素克隆时会被保留）。
+ * 配置 title 函数，从 data-original-url 派生文件名。
+ *
+ * 关键点（已核对 viewer.min.js 源码）：title 回调拿到的 `image` 是 Viewer 内部新建的 <img>
+ * （view(): `var image = document.createElement('img')`），不是原始 <div>。Viewer 在构建缩略图
+ * 与查看大图时只透传 `inheritedAttributes` 里的属性，默认值不含 data-original-url，所以必须把
+ * data-original-url 加进 inheritedAttributes，否则 title 函数永远读不到它、标题恒为 'image'。
  */
 public class PictureFilePreviewTests {
 
@@ -35,6 +40,13 @@ public class PictureFilePreviewTests {
                 () -> "picture.ftl must derive the title from data-original-url (see #787)");
         assertTrue(pictureTemplate.contains("getAttribute('data-original-url')"),
                 () -> "title derivation must read data-original-url attribute (see #787)");
+
+        // 关键：Viewer.js 只会把 inheritedAttributes 透传到 title 回调拿到的 <img> 上，默认值不含
+        // data-original-url。若不显式加入，title 函数读不到原始 URL、标题恒为 'image'（修复形同虚设）。
+        assertTrue(pictureTemplate.contains("inheritedAttributes"),
+                () -> "picture.ftl must set Viewer inheritedAttributes so data-original-url reaches the title image (see #787)");
+        assertTrue(pictureTemplate.contains("'data-original-url'"),
+                () -> "Viewer inheritedAttributes must include data-original-url (see #787)");
     }
 
     private String readResource(String resourcePath) throws IOException {