Fix/clip line number gutter to match editor viewport height

[rachel-d-07]

Description

Line numbers in the code editor continued rendering outside the
bordered editor container when code exceeded ~22 lines.

Fixed by using requestAnimationFrame to sync the line numbers div
height to the editor's clientHeight after each render cycle,
preventing overflow beyond the visible editor boundary.

Related Issue

Fixes #403

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

Before

After

Test evidence

This is a frontend-only JavaScript fix. No backend changes were made.
pytest is not applicable for this change.

Manual testing performed:

• Loaded sample code (28 lines) → line numbers stay inside editor border
• Typed 30+ lines manually → no overflow below editor boundary
• Tested in Chrome on Windows