Split player.html into separate JS and CSS files

[paulpv]

Summary

The single player.html file currently mixes HTML structure, embedded styles, and application logic. For maintainability and to align with our UI guidelines, let's extract the inline script and styles into dedicated .js and .css files while keeping the app functioning as a static build.

Tasks

• Move the inline JavaScript into a new module (e.g. player.js) and update player.html to load it.
• Move the inline styles into a new stylesheet (e.g. player.css) and reference it from player.html.
• Ensure relative paths keep the project self-contained and offline-friendly.
• Update the README with any new development or manual testing steps if needed.
• Manually verify player functionality (folder selection, playback, persistence) in Chromium-based browsers.

Acceptance Criteria

• player.html contains minimal markup with external references to the new JS and CSS assets.
• The player loads and behaves exactly as before after the refactor.
• README instructions remain accurate and up to date.