Fix lesson print layout

[Sinkleberg]

Summary

Adds a lesson print layout so saved PDFs focus on the lesson instead of the site navigation. Closes #32.

Type of change

• Website / build bug fix

What changed

• Hide navigation and controls, use readable print colors in either theme, and print external link URLs.
• Wrap long code, keep ordinary code blocks and headings together where possible, and preserve RTL alignment.

How it was tested

• node --test: 46 passed.
• node site/build.js: passed.
• Chromium PDF checks: light/dark themes, a real code-heavy lesson, oversized code blocks, and RTL alignment.

Other browser print engines haven't been checked.

Screenshots

Before and after: the same lesson printed to PDF.

Before	After

Contributor checklist

• One focused stylesheet change; no lesson, curriculum, translation, or dependency changes.
• No unfinished placeholders added.
• Read the contributing guide and code of conduct.