Keep iOS tablet header compact and remove duplicate search

[kasnder]

Summary

Follow-up to #33, which merged before the final header review was finished.

• Restore the compact tablet header: logo, search form, and language switcher share the first row where they fit.
• On phones, keep the Search navigation link and hide the duplicate header form, reducing header height.
• On wider subpages, keep the search form and hide the redundant Search link.

Verification

• git diff --check
• Reviewed the two-file diff against merged main.
• The repository's CI runs the full Node test suite.

Review note

The updated layout could not be rendered locally because this worktree has no installed pug dependency and the browser blocked a local file preview. Please check phone and tablet widths in a deployment preview before merging.