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 @gib/ui modules they import, and client code reads public env from publicEnv.ts so zod stays out of the browser.

apps/next/src/components/layout/auth/profile/sign-out.tsx

@@ -2,6 +2,7 @@
 
 import { useState } from 'react';
 import { useRouter } from 'next/navigation';
+import { resetAnalyticsIdentity } from '@/lib/analytics';
 import { useAuthActions } from '@convex-dev/auth/react';
 import { LogOut } from 'lucide-react';
 
@@ -22,6 +23,7 @@ export const SignOutForm = () => {
     setIsSigningOut(true);
     try {
       await signOut();
+      resetAnalyticsIdentity();
       router.push('/');
     } catch (error) {
       console.error('Sign out error:', error);