feat: add logo to README and social preview asset

Move logo.png from root to assets/, optimized to 202KB (README) and
499KB (social preview at GitHub's recommended 1280x640). Display
centered at top of README above badge rows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: travisjneuman

README.md

@@ -1,5 +1,9 @@
 # 🐍 Learn Python — Zero to World-Class Full-Stack Developer
 
+<p align="center">
+  <img src="./assets/logo.png" alt="Learn Python" width="600">
+</p>
+
 <p align="center">
   <img src="https://img.shields.io/badge/Python-3.11+-3776AB?style=for-the-badge&logo=python&logoColor=white" alt="Python 3.11+">
   <img src="https://img.shields.io/badge/Projects-251-2ea44f?style=for-the-badge" alt="251 Projects">