Add ability to refresh/re-extract weblink's preview image

As a follow-up to my last bug report, it would be helpful to add the option to refresh a weblink's preview image, to fix those that fail to extract it when the weblink was first saved, without having to re-save it.

Capacities v1.72.3 (Nightly); none none; Mac OS v10.15.7; Chrome v148.0.7778.280 (WebKit v537.36); Desktop (Electron); 27/10/2026, 15:40:44