fix: bump contrast on attachment overlay text

[KeeJef]

This is a proxy pull request from the following codeberg repo, https://codeberg.org/gravel/session-desktop/commits/branch/more-images-count-visibility

When viewing a message with too many image attachments,
a preview of the remaining images is shown,
overlaid with an indicator of the remaining image count.

The indicator was, however, poorly visible on light image backgrounds.

This commit increases the text size of the indicator and includes
a drop shadow on the indicator text, thus improving the indicator's
readability.

Relates: to #2762

[Bilb]

Would be good to have a screenshot of how it looks after the change, but I'll give it a try locally.

[Bilb]

I am not sure this is going to be enough. Can you force the background to be darker with something like this instead on the

[...]
 background-color: rgba(0, 0, 0, 0.7);

it looks like this

[Bilb]

will check with design team

[Bilb]

@KeeJef design said we want background-color: rgba(0, 0, 0, 0.7); no text-shadow and font size back to 20xp. Can I let you update this PR or I make one on the side?

[KeeJef]

I can update

[KeeJef]

Updated

[Bilb]

lgtm