fix(ui): prevent media drawer thumbnails from eager preloading

[mattiaz9]

What?

Updates thumbnail rendering to avoid eager image preloading and instead let the browser load thumbnails directly with loading="lazy" / decoding="async". Also adds handling for cached images so thumbnails do not stay in a loading state when drawers are reopened.

Why?

When selecting media with cacheComponents enabled, opening a media drawer with multiple preview thumbnails could trigger refresh/reload behavior and cause unsaved form changes to be lost. Disabling displayPreview avoided the issue, pointing to thumbnail loading as the trigger.

How?

Removes the manual new Image() probe from the thumbnail component, renders the <img> directly, and tracks load/error state through the image element. Cached images are detected via complete / naturalWidth so reopening the drawer does not leave thumbnails showing an indefinite shimmer.

Fixes #16270

[mattiaz9]

I just realized the version in main is v4-beta.
If you plan to merge this please do it in v3, it's almost unusable with cacheComponents enabled.