Add theme-aware viralens README posters (#4)

* Add theme-aware README artwork

* Add light and dark viralens posters

Author: HarryXin0919

README.md

@@ -1,6 +1,10 @@
 <div align="center">
 
-<img src="banner.svg" alt="viralens" width="100%">
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="reports/img/viralens-github-dark.png">
+  <source media="(prefers-color-scheme: light)" srcset="reports/img/viralens-github-light.png">
+  <img src="reports/img/viralens-github-light.png" alt="viralens creator analytics overview" width="100%">
+</picture>
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-FD4E63.svg)](LICENSE)
 &nbsp;[![Download](https://img.shields.io/badge/⬇%20Download-Win%20·%20Mac%20·%20Linux-2EA44F)](https://github.com/HarryXin0919/viralens/releases/latest)