Add lib/google-ads.ts (lazy gtag init, gclid capture, Enhanced Conversions, trackGoogleAdsConversion). Fire sign-up conversion on email registration and purchase conversion on Stripe success. Gated on VITE_GOOGLE_ADS_* env; no-op when unset. Mirrors the Meta Pixel/CAPI pattern. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>