Align embed UI with standalone player

[damons]

Summary

• restyle public/embed.js to use the same visual language as the standalone player
• replace text buttons with the same icon-style play/mute controls and add a volume slider
• match the standalone off-air and error presentation more closely while keeping the existing embed API intact

Why

The README recommends the script embed path, but its UI currently looks materially different from the standalone player shipped in the same release. This makes the recommended integration feel like a different product.

Testing

• node --check public/embed.js
• rebuilt the player into a static site bundle and loaded both /embed-test.html and / in headless Chrome
• verified the embed renders the same core card layout, status pill, media column, icon controls, and volume slider as the standalone player