🚀 Feature: Have images fade in only after they've loaded

### Bug Report Checklist

- [X] I have tried restarting my IDE and the issue persists.
- [X] I have pulled the latest `main` branch of the repository.
- [X] I have [searched for related issues](https://github.com/JoshuaKGoldberg/dot-com/issues?q=is%3Aissue) and found none that matched my issue.

### Overview

Following #215 & #258: I didn't have time to keep in support for fading images after they've loaded. But I'd really like to.

Loading joshuakgoldberg.com on a simulated Slow 3G connection shows the image taking a while then suddenly appearing in:

https://github.com/JoshuaKGoldberg/dot-com/assets/3335181/80945486-32f7-4af6-b6ca-5488a03e4303

I find this jarring. But even if I didn't find it jarring, I still think it looks cool to have images fade in after the rest of the page. It's a nifty effect.

### Additional Info

_No response_