Rework PostHog setup — use Next.js ingest proxy + posthog-js/react

Mirrors the proven pattern from gendersloty: rewrites /ingest/* to
the PostHog host so requests go through the same origin (no CORS,
no ad-blocker issues). Uses PostHogProvider and usePostHog from
posthog-js/react official React integration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/posthog.ts

@@ -1,2 +1,3 @@
 import posthog from 'posthog-js'
 export { posthog }
+export { usePostHog } from 'posthog-js/react'