Integrate PostHog across the Next.js website and calculator

[anth-volk]

Fixes #919

Summary

This branch integrates PostHog into the new Next.js website and calculator-app surfaces and adds the shared calculator analytics layer those apps consume from app/src.

At a high level, it:

• bootstraps PostHog in both Next apps using Next.js integration points
• adds typed calculator analytics helpers, event schemas, and snapshot builders
• instruments key calculator flows across report building, household creation, calculations, and report output
• instruments key website interactions such as entering the calculator, newsletter signup, research interactions, and country switching
• adds stronger client, boundary, and server-side error capture
• proxies PostHog traffic through the Next apps instead of sending browser traffic directly to PostHog ingestion hosts
• adds release-aware env/config scaffolding and pins the repo to the supported Node version via .nvmrc / .node-version

Included

Next app bootstrap and monitoring

• website/instrumentation-client.ts
• website/instrumentation.ts
• website/src/app/error.tsx
• website/src/app/global-error.tsx
• website/src/lib/posthog-server.ts
• website/src/lib/posthogProxy.ts
• calculator-app/instrumentation-client.ts
• calculator-app/instrumentation.ts
• calculator-app/src/app/error.tsx
• calculator-app/src/app/[countryId]/error.tsx
• calculator-app/src/app/global-error.tsx
• calculator-app/src/lib/posthog-server.ts
• calculator-app/src/lib/posthogProxy.ts

Shared calculator analytics

• typed event payloads in app/src/utils/analyticsSchemas.ts
• snapshot builders in app/src/utils/analyticsSnapshots.ts
• shared capture helpers in app/src/utils/analytics.ts
• shared browser/client access in app/src/utils/posthogClient.ts
• shared error helpers in app/src/utils/errorTracking.ts

Calculator instrumentation

• report builder opened / started / created
• report-builder year, policy, and population selection events
• household builder opened / variable add-remove / save
• calculation started / completed / failed
• report output viewed / subpage viewed
• output actions like chart export and Python reproduction copy

Website instrumentation

• enter-calculator CTA
• newsletter signup started / succeeded / failed
• research article viewed
• research filters changed
• country switched

Validation

• app: bun run typecheck
• website: bun run typecheck and bun run build
• calculator-app: bun run typecheck and bun run build

These were validated under nvm use with the repo pinned to Node 24.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
policyengine-app-v2	Building	Preview	Apr 6, 2026 2:52pm
policyengine-calculator	Ready	Preview, Comment	Apr 6, 2026 2:52pm
policyengine-calculator-next	Error		Apr 6, 2026 2:52pm
policyengine-website	Ready	Preview, Comment	Apr 6, 2026 2:52pm