Fix: add waveform-is-placeholder class to container on analysis failure

[Intenzi]

Closes #19

Summary

This PR adds a CSS state class waveform-is-placeholder to the player container when the waveform generation fails and falls back to placeholder peaks (such as on CORS blocked tracks).

Details

• Removes the .waveform-is-placeholder class at the start of load() to reset the player state when loading new tracks.
• Adds the .waveform-is-placeholder class inside the load() catch block when analysis fails.
• Adds unit tests in test/player.test.js to verify class toggling on load and failure.

Why this is needed

This class allows integrating applications (such as the WordPress Gutenberg editor) to identify when a decorative waveform is being shown and conditionally restyle it (e.g. flat-lining it to look like a standard progress bar instead of displaying randomised audio peaks).

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Gemini 3.5 Flash (Low)
Used for: Help in identifying and locating the relevant code lines to fix, approach of fixing. Tests were implemented via it. Final implementation was reviewed and tested by me.

[arraypress]

Thanks @Intenzi — clean, well-tested contribution. Merged and released in 1.24.0, live on npm now, and the new class is documented on the styling page.