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.
Introduce a new $quality template variable that is only substituted into filenames to avoid splitting album folders when tracks of mixed qualities are present. Updates include:
- web_server.py: populate template contexts with 'quality' (from context['_audio_quality']), strip $quality from folder components, substitute it only in the filename, and clean up empty brackets/parentheses/dashes when the variable is empty.
- config/config.example.json and config/config.json: document the new variable in the file_organization template variables string.
- webui/index.html and webui/static/script.js: update UI help text and client-side template validation to include $quality.
This prevents folder fragmentation for albums with mixed-quality files while still allowing quality information in filenames.
