fix: merge checkout events onto storefront distinct ids

[apps3000]

Summary

• Reads _posthog_distinct_id from Shopify checkout/cart attributes passed in from the storefront.
• Bootstraps PixieHog's PostHog client with the storefront distinct id when it is already present on init cart data.
• For checkout events, sets the PixieHog distinct id to the storefront id before capture, then identifies that same id with checkout/customer email properties.
• Keeps the existing email-based fallback only when no storefront distinct id is available.

Why

Tuftinglove's Hydrogen storefront writes the PostHog storefront distinct_id into the Shopify cart as _posthog_distinct_id. PixieHog previously used its own Web Pixel distinct id and later identified by email, so checkout events often landed on a separate PostHog person. Using the storefront id directly lets checkout events merge into the existing storefront person.

Verification

• npm run build
• npx eslint extensions/web-pixel/src/index.ts extensions/web-pixel/src/storefront-identity.ts
• npx tsc --noEmit --pretty false shows no extensions/web-pixel errors; existing unrelated app settings type errors remain in app/routes/app.web-pixel-settings/default-web-pixel-settings.ts.

Notes

• Direct push to celadonworks/pixiehog-app was not permitted for the apps3000 account, so this PR is opened from the apps3000/pixiehog-app fork.