fix(site): align install requirements grid responsively

[huyua9]

What changed

• use three columns at the md breakpoint and five at lg for the five install requirements
• keep mobile row dividers and align row/column borders at responsive breakpoints
• avoid an orphaned fifth card and stray divider at the md row break

Why

The live install page already has five requirements but still uses md:grid-cols-4 md:divide-y-0, which leaves the fifth requirement alone on a second row without a matching horizontal boundary.

Validation

• git diff origin/main...HEAD --check
• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/tailwindcss -i ./app/globals.css -o /tmp/sendo-install-grid.css --minify
• next build attempted; blocked by the environment failing to fetch the existing Google Font (IBM Plex Mono) from Google Fonts

AI-assisted; the change is intentionally limited to apps/site/app/install/page.tsx.

[vercel]

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