Fix mobile search results panel alignment while scrolling

[bunny953]

Description

Fixes #309

The mobile search results panel could become detached from the search bar when the mobile navigation container was scrolled.

This change:

• Positions the search results panel relative to the viewport.
• Keeps the panel aligned with the search input during scrolling.
• Repositions the panel when the mobile offcanvas container scrolls.
• Prevents the panel from overflowing the viewport on smaller screens.

Testing

• node --check theme/js/osl-search.js
• git diff --check
• Verified the site runs successfully with makim pages.preview.

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[bunny953]

Hi! The PR's CI run currently shows the linkcheckmd hook failing, although I reproduced the same check locally with:

pre-commit run linkcheckmd --all-files

and it passed successfully.

The other relevant checks (prettier, ruff-format, and ruff-linter) also passed in the CI run. Please let me know if you'd like me to investigate anything further.