chore: NEXT_PUBLIC_GA_ID is declared but never actually used

[LarytheLord]

lib/env.ts declares NEXT_PUBLIC_GA_ID as an optional env var, but no gtag/Google Analytics script tag exists anywhere in the codebase. Setting a real ID in Vercel currently does nothing — only Vercel's own Analytics (page views/web vitals) is live, which is not a full GA/Search-Console-linked property.

Acceptance Criteria

• Either wire up a real GA4 script using NEXT_PUBLIC_GA_ID in app/layout.tsx, or remove the unused env var declaration to avoid the false impression that analytics is configured