fix(prism): apply syntax highlighting to dynamically created code elements

[developerfred]

Summary

Fixes #8 - Prism.js styles not applying on form submit.

When users change secondary languages and click Compare, the syntax highlighting styles were not applied until page reload.

Changes

• Changed code element classes from custom-${lang} to language-${lang} to match Prism.js expected pattern
• Added Prism.highlightAllUnder(section) in createLiSection() to apply highlighting to dynamically created elements
• Optimized to apply highlighting only to specific sections instead of re-highlighting the entire page

Technical Details

The fix addresses two issues:

1. Prism.js only recognizes classes matching the pattern language-xxxx, not custom-xxxx
2. Prism.highlightAll() was not being called after dynamic element creation

By using Prism.highlightAllUnder(section) within createLiSection(), highlighting is applied efficiently to newly created elements only, improving performance.

Closes #8