Introduce $artistletter and $disc template variables across config, UI, and backend to support artist-first-letter tokens and multi-disc albums. Update web_server.py to include disc_number in template context, prefer user-controlled $disc in templates, and create configurable disc subfolders using a new file_organization.disc_label setting. Update example and active config, web UI to expose the new variable and disc label selector, and script.js to validate, load, and save the new settings and substitutions.
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