Add visitor map widget, categorize publications by research_category, and improve bib rendering

[zuojr]

Motivation

• Provide an optional live visitor map widget for the site and expose configuration for it in _config.yml.
• Organize bibliography entries by a new research_category field so publications can be grouped on the publications page.
• Improve bibliography rendering and hide custom bibtex fields from public output.

Description

• Added visitor_map configuration to _config.yml, a new _includes/visitor_map.liquid include, CSS in _sass/_components.scss, and included the widget on the about page to support an embeddable ClustrMaps visitor map.
• Extended bibliography entries in _bibliography/papers.bib with research_category metadata and added several new entries and fields (e.g. arxiv, pdf, code, doi).
• Updated _pages/publications.md to render separate publication groups via research_category, and added .publication-category styles in _sass/_publications.scss.
• Enhanced _layouts/bib.liquid to show a generic "Paper" link when url is present and no arxiv/pdf/html is provided.
• Modified _plugins/hide-custom-bibtex.rb to read filtered_bibtex_keywords from either the root config or scholar section and cleaned up matching logic to strip configured custom fields from displayed BibTeX.
• Toggled selected_papers to false on the home about page to avoid duplication with the new publications grouping.

Testing

• Ran a local site build with jekyll build, which completed successfully with the updated includes and templates.

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Codex Task

[github-actions]

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