Add research-by-problem page and navigation link

[PhilippPelz]

Note

Low Risk
Low risk documentation/content change that only adds a new static page and navigation links; no application logic or data handling is modified.

Overview
Adds a new research-by-problem.qmd page that presents lab research organized by problem areas, with breadcrumbs and representative publication links.

Updates site navigation in _quarto.yml to include a By Problem navbar item, and adds a cross-link from research.qmd to the new problem-first view.

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