fix(#787): show filename in image preview title via data-original-url

[wylovelyi]

Fixes #787.

Direct image previews use hidden <div src> elements, so Viewer's default title cannot obtain a filename from the DOM src property. Derive the title from the selected original element's data-original-url, stripping query/fragment parts before extracting the filename.

The callback reads this.images[this.index]: Viewer replaces data-original-url on its internal image clones with the actual fetch URL, which is getCorsFile?... when kkagent=true. Reading the original element preserves the source filename in both direct and proxy previews.

Validation:

• PictureFilePreviewTests passes locally.
• Browser checks against the bundled Viewer confirm photo.png with kkagent=false and kkagent=true; the previous proxy case displayed getCorsFile.
• Added picture-title.spec.ts to verify the rendered title through /onlinePreview in both modes.

[wylovelyi]

@klboke 你好，这个 PR 在图片预览标题里通过 data-original-url 显示文件名（#787）。准备好 review 了，谢谢！

[wylovelyi]

Pushed a fix after verifying the original approach against the bundled viewer.min.js (in static/js/viewer.min.js).

Root cause (confirmed by reading the source): the title(image, imageData) callback does NOT receive the original <div>. Viewer.js creates a brand-new <img> internally (view(): var image = document.createElement('img')) and passes that to the callback. Viewer only forwards the attributes listed in its inheritedAttributes option when building that <img>, and data-original-url is NOT in the default list (['crossOrigin','decoding','isMap','loading','referrerPolicy','sizes','srcset','useMap']). So the previous getImageNameFromOriginalUrl(image) always read null and the title stayed 'image' — the fix was a no-op at runtime (exactly the review concern).

Fix: add 'data-original-url' to the Viewer inheritedAttributes so the original URL is forwarded from the thumbnail <img> to the title <img>. The existing getImageNameFromOriginalUrl now resolves the real filename (not the CORS-proxy URL). Also updated PictureFilePreviewTests to assert inheritedAttributes includes data-original-url so this regression cannot silently return.

[klboke]

Reviewed the latest head 86c2b8d, including the follow-up proxy fix.

The bundled Viewer rewrites data-original-url on its image clones, so copying that attribute through inheritedAttributes still showed getCorsFile when kkagent=true. The callback now reads the selected original element via this.images[this.index], retaining the original filename in both modes. I reproduced the previous failure and verified the corrected title in a real browser against the bundled Viewer.

Validation: PictureFilePreviewTests passes locally; the new browser E2E checks for direct and proxy image titles both pass. Linux/Windows/macOS packaging and the full PR E2E job are green on this exact head. No remaining blockers.