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>