Restore syntax highlighting

[AndreyVMarkelov]

Summary

• restore syntax highlighting for curl, Python, HTTP, and JSON output
• highlight source before React renders it, avoiding DOM mutations that caused stale request views
• bundle the current highlight.js theme locally
• add regression coverage for highlighted updates and the existing Host header test

Impact

Generated requests and API responses are readable with syntax coloring again, while parameter and format changes continue to update correctly.

Fixes #27.

Validation

• npm run lint
• npm run typecheck
• npm test
• npm run build
• npm audit --audit-level=moderate
• git diff --check
• manually verified format and parameter updates in the local explorer

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.65%. Comparing base (7beb365) to head (ccb4646).

Files with missing lines	Patch %	Lines
src/utils.ts	90.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #337      +/-   ##
==========================================
- Coverage   14.26%   12.65%   -1.62%     
==========================================
  Files           6        6              
  Lines        5775     5792      +17     
  Branches       28       36       +8     
==========================================
- Hits          824      733      -91     
- Misses       4950     5058     +108     
  Partials        1        1              

Flag	Coverage Δ
unit	12.65% <90.00%> (-1.62%)	⬇️

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