Add visitor map widget and improve publications/bibtex handling

[zuojr]

Motivation

• Provide a live visitor map widget for the site and allow authors to better organize publications by research category.
• Expose a simple way to hide custom BibTeX fields and surface fallback "Paper" links when no PDF/arXiv/HTML is present for an entry.

Description

• Add a configurable visitor map feature with site-level settings in _config.yml (visitor_map block) and a new include _includes/visitor_map.liquid that embeds a ClustrMaps widget or a placeholder.
• Include the visitor map on the about page by updating _layouts/about.liquid and add styles for the widget in _sass/_components.scss.
• Add publication grouping by research_category in _pages/publications.md and add publication-category styles in _sass/_publications.scss.
• Extend papers.bib with new entries and research_category fields for grouping and selection, and mark some entries as selected/displayed appropriately.
• Show a Paper button in _layouts/bib.liquid when an entry has a url but no arxiv, pdf, or html set, and keep existing DOI/arXiv/PDF/HTML buttons behavior.
• Make the custom BibTeX-hiding plugin _plugins/hide-custom-bibtex.rb read filtered_bibtex_keywords from either top-level config or scholar.filtered_bibtex_keywords and preserve the previous superscript-cleaning behavior.
• Disable selected_papers on the about page (_pages/about.md) to avoid duplicate selected publications display.

Testing

• Ran a local site build with bundle exec jekyll build and the build completed without errors.
• Checked Ruby syntax for the plugin with ruby -c _plugins/hide-custom-bibtex.rb, which returned OK.
• Manually inspected generated publication pages and the about page to verify the visitor map include appears when enabled and that grouped bibliographies render expected entries.

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Codex Task

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