README & homepage: add npm usage, GitHub link with logo, analytics

[exisz]

DoD
README.md: Add npm install/usage section, badges (npm version, downloads, license), link to isitstable.com

Homepage (page.tsx): Add GitHub link with GitHub logo/icon, npm package link with usage example

Analytics: Wire up Vercel Analytics (or GA4) to isitstable.com — add @vercel/analytics to layout.tsx

Acceptance
README has install command, badges, quick usage

Homepage has visible GitHub icon+link and npm link

Analytics collecting pageviews on production

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Migrated from NEBULA-98 | Parent: NEBULA-84 | Original status: Engineering