🚀 Feature: Switch light/dark mode toggle from emoji to actual icon

### Feature Request Checklist - [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 The emoji has always bugged me as looking not in line with the rest of the site's theme. <img width="1043" height="222" alt="Screenshot of the top of joshuakgoldberg.com in light mode, showing a 🌞 emoji" src="https://github.com/user-attachments/assets/c0f1d08e-5e53-49fe-b1ea-809506d7949a" /> ### Additional Info 💖