feat: add PostHog, move Plausible to v4, and drop Sentry session replay

PostHog loads after the page is idle through a same-origin /ingest proxy and follows the signed-in Convex user. Plausible uses the per-site script through a /pa proxy. Sentry keeps errors and traces only. Pages load only the @spoon/ui modules they import, and client code reads public env from publicEnv.ts so zod stays out of the browser. Both analytics integrations stay off until their env is set in Infisical prod.

docker/Dockerfile

@@ -10,7 +10,9 @@ ARG SENTRY_AUTH_TOKEN
 ARG SENTRY_DISABLE_AUTO_UPLOAD=false
 ARG NEXT_PUBLIC_SITE_URL
 ARG NEXT_PUBLIC_CONVEX_URL
-ARG NEXT_PUBLIC_PLAUSIBLE_URL
+ARG NEXT_PUBLIC_PLAUSIBLE_SRC
+ARG NEXT_PUBLIC_POSTHOG_KEY
+ARG NEXT_PUBLIC_POSTHOG_HOST
 ARG NEXT_PUBLIC_SENTRY_DSN
 ARG NEXT_PUBLIC_SENTRY_URL
 ARG NEXT_PUBLIC_SENTRY_ORG
@@ -21,7 +23,9 @@ ENV SENTRY_AUTH_TOKEN=$SENTRY_AUTH_TOKEN
 ENV SENTRY_DISABLE_AUTO_UPLOAD=$SENTRY_DISABLE_AUTO_UPLOAD
 ENV NEXT_PUBLIC_SITE_URL=$NEXT_PUBLIC_SITE_URL
 ENV NEXT_PUBLIC_CONVEX_URL=$NEXT_PUBLIC_CONVEX_URL
-ENV NEXT_PUBLIC_PLAUSIBLE_URL=$NEXT_PUBLIC_PLAUSIBLE_URL
+ENV NEXT_PUBLIC_PLAUSIBLE_SRC=$NEXT_PUBLIC_PLAUSIBLE_SRC
+ENV NEXT_PUBLIC_POSTHOG_KEY=$NEXT_PUBLIC_POSTHOG_KEY
+ENV NEXT_PUBLIC_POSTHOG_HOST=$NEXT_PUBLIC_POSTHOG_HOST
 ENV NEXT_PUBLIC_SENTRY_DSN=$NEXT_PUBLIC_SENTRY_DSN
 ENV NEXT_PUBLIC_SENTRY_URL=$NEXT_PUBLIC_SENTRY_URL
 ENV NEXT_PUBLIC_SENTRY_ORG=$NEXT_PUBLIC_SENTRY_ORG