Fix HTML sanitization for formulas

[atsyplenkov]

This broadens HTML sanitization from table/img fragments to allowlisted HTML regions while preserving code spans, malformed HTML, and TeX math content.

Closes #4.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 90.51724% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.21%. Comparing base (aaac223) to head (a579b31).
⚠️ Report is 48 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/markdown.rs	90.82%	21 Missing ⚠️
src/core.rs	66.66%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   85.45%   86.21%   +0.76%     
==========================================
  Files           8        8              
  Lines         990     1197     +207     
==========================================
+ Hits          846     1032     +186     
- Misses        144      165      +21     

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