fix(docs): restore visible search results across screen sizes

[singaraiona]

What & why

Documentation search opened its input but hid the result list: the custom header clipped the absolutely positioned desktop results, and its backdrop filter constrained the fixed mobile panel to the header height. Allow the open results to extend below the form and remove the containing backdrop filter while mobile search is open.

Exclude Material search-result articles from the site's display typography so results keep compact headings and snippets instead of inheriting page-sized headings and the documentation banner.

Validation:

• mkdocs build --strict passes.
• Chromium checks at 1440px, 1024px, and 390px: open search, type select, verify visible and clickable results, and follow the first result to Select Queries. No JavaScript errors.
• Visually checked desktop and mobile results.

Checklist

• PR targets dev (not master)
• Commits follow Conventional Commits
• Documentation builds cleanly with mkdocs build --strict
• Browser validation passes; C tests are not applicable to this CSS change