Update README and index.html for enhanced metadata and visuals. Added a thumbnail link to README and improved SEO with Open Graph and Twitter meta tags in index.html, including updated title and description for better social sharing.

Add ambience generation features and assets. Introduced ambience.py for procedural and sampled ambience beds, updated app.py to integrate ambience selection into music generation, and modified requirements.txt to include new dependencies. Added scripts for fetching and rendering ambience samples, along with new audio assets and credits for attribution.