- Enable capture_exceptions (unhandled errors + rejections) in the PostHog
  provider and tag every event with an `environment` super-property derived
  from NEXT_PUBLIC_VERCEL_ENV (prod/preview/dev).
- Report React render errors via posthog.captureException in the shared
  ErrorBoundary, and wrap the dashboard layout in a PostHogErrorBoundary.
- Upload source maps during the build via @posthog/nextjs-config
  (deleteAfterUpload), gated on POSTHOG_API_KEY, release = commit SHA.
- Add POSTHOG_API_KEY / POSTHOG_PROJECT_ID / NEXT_PUBLIC_VERCEL_ENV to the
  env schema and .env.example.
- Mask the raw error message in the shared ErrorBoundary and vertically
  center the dashboard route error.

…capture

- Only initialize PostHog on /dashboard routes (lazy init, guarded against
  re-init) so auth, marketing, and proxied (docs/blog) paths aren't tracked.
- Guard the shared ErrorBoundary's captureException on a live client
  (posthog.__loaded) instead of the env key, so it no-ops where PostHog isn't
  initialized rather than dropping events on an uninitialized singleton.
- global-error renders outside the provider tree, so it now initializes its own
  minimal client and reports root-layout/provider failures explicitly
  (ErrorBoundary report={false} avoids double-capture).