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/app/(frontend)/layout.tsx

@@ -36,34 +36,38 @@ const RootLayout = ({
 }: Readonly<{
   children: React.ReactNode;
 }>) => {
+  const app = (
+    <ThemeProvider
+      attribute='class'
+      defaultTheme='system'
+      enableSystem
+      disableTransitionOnChange
+    >
+      <ConvexClientProvider>
+        <div className='flex min-h-screen flex-col'>
+          <Header />
+          {children}
+          <Footer />
+        </div>
+        <Toaster />
+      </ConvexClientProvider>
+    </ThemeProvider>
+  );
   return (
     <ConvexAuthNextjsServerProvider>
-      <PlausibleProvider
-        domain={env.NEXT_PUBLIC_SITE_URL.trim().replace(/^https?:\/\//, '')}
-        customDomain={env.NEXT_PUBLIC_PLAUSIBLE_URL}
-      >
-        <html lang='en' suppressHydrationWarning>
-          <body
-            className={`${geistSans.variable} ${geistMono.variable} antialiased`}
-          >
-            <ThemeProvider
-              attribute='class'
-              defaultTheme='system'
-              enableSystem
-              disableTransitionOnChange
-            >
-              <ConvexClientProvider>
-                <div className='flex min-h-screen flex-col'>
-                  <Header />
-                  {children}
-                  <Footer />
-                </div>
-                <Toaster />
-              </ConvexClientProvider>
-            </ThemeProvider>
-          </body>
-        </html>
-      </PlausibleProvider>
+      <html lang='en' suppressHydrationWarning>
+        <body
+          className={`${geistSans.variable} ${geistMono.variable} antialiased`}
+        >
+          {/* Plausible loads its per-site script through the same-origin
+              proxy in next.config.js; without that config it stays off. */}
+          {env.NEXT_PUBLIC_PLAUSIBLE_SRC ? (
+            <PlausibleProvider>{app}</PlausibleProvider>
+          ) : (
+            app
+          )}
+        </body>
+      </html>
     </ConvexAuthNextjsServerProvider>
   );
 };