Add product preview to README

[rimtty]

Summary

• add the supplied QuickPods product overview image near the top of the main README
• store the image under docs/assets/screenshots so the README does not depend on an external host
• add the same preview to the Japanese README with localized alternative text

User impact

Visitors can understand the Bluetooth audio flyout, taskbar controls, and Settings experience before reading the feature list.

Validation

• verified that both README files reference the repository image
• verified the copied PNG dimensions (1672 × 941), size, and SHA-256 against the supplied source
• build/Test-RepositoryPublicReadiness.ps1 — passed
• git diff --check — passed