[codex] Fix header menu overflow

[redreceipt]

Summary

• Strengthen the site-menu dropdown selector so it overrides Pico's default left-aligned dropdown positioning.
• Add a viewport max width to keep the header menu inside the page.
• Add regression coverage for the local-pages menu override.

Root Cause

The hamburger menu was placed near the right edge of the header, but Pico's default dropdown rule could still apply left: 0, causing the menu to expand off the right side of the viewport.

Validation

• source venv/bin/activate && ruff check .
• source venv/bin/activate && python -m unittest discover -s tests -p 'test_*.py'
• Playwright rendered check at 1280px and 390px widths confirmed the opened dropdown stayed inside the viewport.