fix: url input is hidden after first keystroke

[jxjj]

fixes #104

On the card edit/create page, the audio block will hide the input field once there's some text in it. This is fine if copy/pastes a url, but if they're typing a url for some reason the user won't be able to continue after first character. Additionally, whether the url is valid or not an audio player shows.

This changes the block to be similar to the image block:

• the url field is always visible
• invalid urls will show an error, not try rendering an audio component