feat(embed): YouTube audio-only option in posts
Per embed you now choose audio-only or video. [[embed:audio:URL]] (YouTube)
renders as an audio card with our controls (play/seek/volume + thumbnail as
cover); the YT player runs hidden off-screen and provides only the sound.
[[embed:URL]] stays video. The editor embed button asks for audio or video
when a YouTube URL is given. Server: audio:/video: prefix in the shortcode
→ data-embed-mode. busters embed-player.js?v=8, embed.css?v=5.
Co-Authored-By: Claude <noreply@…>