Chore: Set max thumbnail width to home/explore image max width

[RoccoSmit]

Images in the web client have a maximum width of 700px.

This PR is to update the width of thumbnails being generated to align with the home/explore page image max width. This will allow the images to take up the full space provided

Images with a width smaller than the thumbnail width are not scaled up. Note: for these images the cache and resource will be the same image.

See PR comment for original discussion: https://github.com/usememos/memos/pull/3821#discussion_r1731674073

e.g.

original 512px width compared to new 700px width

smaller than 700px width does not get upscaled