docs(readme): left-align install code blocks (were centered by the hero div)

The `<div align="center">` hero centered each line of the code blocks, so shorter
lines (`cd your-project`, `npm i -g …`) rendered with stray leading indentation.
Close the centered hero after the badges; install/init sections render normally.
Only the screenshot stays centered.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>