Avoid automatic remote images in report previews and library picker #908

Problem

Opening an HTML report can automatically request images from external hosts, including CSS background images and images added by scripts. The library picker also requests remote paper cover thumbnails as soon as it displays results.

Expected behavior

Automatic external images should not load by default. Locally referenced report images should remain viewable, and report scripts should continue to work. This is the app-wide media policy described in #430 (comment); the maintainer requested that the HTML preview and library picker changes be reviewed separately from the Mermaid renderer in #898 (comment).

The HTML preview runs report scripts, which may make their own network requests. Blocking automatic image loads alone does not prevent arbitrary script-initiated network access.