Switch mascot to PNG with alpha channel

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: turian

README.md

@@ -1,3 +1,3 @@
 <p align="center">
-  <img src="asset/mascot.jpg" alt="Fastmail JMAP CLI" width="300">
+  <img src="asset/mascot.png" alt="Fastmail JMAP CLI" width="300">
 </p>