Missing images in signature preview

1. Upload an image.
2. Embed the uploaded image into the text.
3. Click on "Show preview".

<img width="613" alt="Image" src="https://github.com/user-attachments/assets/ba85aa16-2ae3-492a-ac69-3cd609a0ecf0" />

ref https://www.woltlab.com/community/thread/311952/