Fix mobile search interactions

[mre]

Summary

• focus the search field synchronously so mobile browsers open the keyboard
• dismiss search on completed clicks instead of pointer-down, preserving results during touch scrolling
• document both mobile interaction constraints

Validation

• tested at a 375x812 mobile viewport in headless Chrome
• confirmed immediate input focus
• confirmed 10 results remain visible after a touch-drag scroll
• confirmed an intentional outside click still closes search
• ran node --check static/search.mjs
• ran zola build
• ran git diff --check