Add ClustrMaps visitor map, bibliographic grouping/filtering, and UI tweaks

[zuojr]

Motivation

• Provide a live visitor map widget and site-wide settings to display visitor tracking on the about page.
• Improve publications listing by grouping entries by research area and exposing a research_category metadata field in the bibliography.
• Make BibTeX rendering more flexible by hiding custom fields from displayed BibTeX and by exposing a direct Paper link when appropriate.

Description

• Add site config visitor_map options to _config.yml and include a new _includes/visitor_map.liquid that embeds the ClustrMaps widget with configurable colors and token.
• Include the visitor map on the about page by updating _layouts/about.liquid and add styles for the widget in _sass/_components.scss.
• Update _pages/publications.md to render two publication groups with jekyll-scholar queries using the new research_category field.
• Extend _bibliography/papers.bib with new entries and annotate many entries with research_category metadata used for grouping.
• Add filtered_bibtex_keywords configuration and update _plugins/hide-custom-bibtex.rb to read that key with a fallback into the scholar namespace, and to continue cleaning superscripts.
• Update _layouts/bib.liquid to show a Paper button when entry.url exists and no arxiv/pdf/html links exist, and add publication category heading styles in _sass/_publications.scss.
• Toggle selected_papers to false in _pages/about.md to avoid duplicating the selected publications block on the landing page.

Testing

• Built the site locally with jekyll build and the build completed without errors.
• Verified that the bibliography rendering flows (grouped queries) and the new Paper button render in generated HTML during the local build.

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Codex Task

[github-actions]

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