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, clear of Payload's /api routes. Sentry keeps errors and traces only. Pages load only the @gib/ui modules they import, and client code reads public env from publicEnv.ts so zod stays out of the browser.

apps/next/src/sentry.server.config.ts

@@ -3,7 +3,8 @@ import * as Sentry from '@sentry/nextjs';
 
 Sentry.init({
   dsn: env.NEXT_PUBLIC_SENTRY_DSN,
-  tracesSampleRate: 1,
+  // One in ten requests keeps traces useful without loading the home server.
+  tracesSampleRate: 0.1,
   enableLogs: true,
   debug: false,
 });