feat(forms): limit secondary languages selection to maximum of 3

[developerfred]

Summary

Implements #7 - Set a max of 3 compare languages.

This feature limits the number of secondary languages users can select to a maximum of 3 (resulting in a 4-column grid: 1 primary + 3 secondary).

Changes

• Added MAX_SECONDARY_LANGUAGES constant set to 3
• Created limitSecondaryLanguages() function to manage checkbox enabled/disabled state
• Added event listeners to all checkboxes to enforce the limit dynamically
• Updated initHomePage() to apply limit after restoring selections from localStorage
• Changed code classes from custom-${lang} to language-${lang} (following Prism.js pattern)

Technical Details

When 3 secondary languages are selected, remaining unchecked checkboxes are disabled. When a user unchecks a language, previously disabled checkboxes are re-enabled. The primary language checkbox remains disabled when selected as primary to prevent duplicate selections.

Closes #7