Generate README Screenshots

Purpose

The README screenshots in docs/img/*.webp are not manually captured — they are deterministically generated from Storybook stories defined in src/browser/stories/App.readmeScreenshots.stories.tsx. When any of these stories change visually (detected by Chromatic or local inspection), the corresponding WebP assets must be regenerated and committed.

Quick Reference

# 1. Build Storybook
make storybook-build

# 2. Serve the static build (keep running in background)
python3 -m http.server 6006 -d storybook-static &