NECTAR-13D fix(mathjax): set explicit CHTML font URLs to prevent CSP violations

[thostetler]

Fixes NECTAR-13D

MathJax CHTML output auto-detects font URLs by inspecting its own
script tag location. When loaded dynamically via better-react-mathjax,
this detection can fail, causing the browser to construct incorrect
font URLs that get blocked by the CSP font-src
directive.

• Set explicit chtml.fontURL in MathJax config tied to each CDN source
• Removed unpkg.com fallback

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.6%. Comparing base (908d3ec) to head (a6d00c6).

Files with missing lines	Patch %	Lines
src/mathjax.ts	86.4%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #796     +/-   ##
========================================
- Coverage    61.6%   61.6%   -0.0%     
========================================
  Files         312     312             
  Lines       35995   36000      +5     
  Branches     1593    1593             
========================================
  Hits        22141   22141             
- Misses      13817   13822      +5     
  Partials       37      37             

Files with missing lines	Coverage Δ
src/mathjax.ts	65.0% <86.4%> (+4.0%)	⬆️

... and 3 files with indirect coverage changes

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