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/env.ts

@@ -20,7 +20,10 @@ export const env = createEnv({
   client: {
     NEXT_PUBLIC_SITE_URL: z.url(),
     NEXT_PUBLIC_CONVEX_URL: z.url(),
-    NEXT_PUBLIC_PLAUSIBLE_URL: z.url(),
+    // Analytics are optional: absent values leave the integration off.
+    NEXT_PUBLIC_PLAUSIBLE_SRC: z.url().optional(),
+    NEXT_PUBLIC_POSTHOG_KEY: z.string().optional(),
+    NEXT_PUBLIC_POSTHOG_HOST: z.url().optional(),
     NEXT_PUBLIC_SENTRY_DSN: z.string(),
     NEXT_PUBLIC_SENTRY_URL: z.string(),
     NEXT_PUBLIC_SENTRY_ORG: z.string(),
@@ -38,7 +41,9 @@ export const env = createEnv({
     PAYLOAD_DB_URL: process.env.PAYLOAD_DB_URL,
     NEXT_PUBLIC_SITE_URL: process.env.NEXT_PUBLIC_SITE_URL,
     NEXT_PUBLIC_CONVEX_URL: process.env.NEXT_PUBLIC_CONVEX_URL,
-    NEXT_PUBLIC_PLAUSIBLE_URL: process.env.NEXT_PUBLIC_PLAUSIBLE_URL,
+    NEXT_PUBLIC_PLAUSIBLE_SRC: process.env.NEXT_PUBLIC_PLAUSIBLE_SRC,
+    NEXT_PUBLIC_POSTHOG_KEY: process.env.NEXT_PUBLIC_POSTHOG_KEY,
+    NEXT_PUBLIC_POSTHOG_HOST: process.env.NEXT_PUBLIC_POSTHOG_HOST,
     NEXT_PUBLIC_SENTRY_DSN: process.env.NEXT_PUBLIC_SENTRY_DSN,
     NEXT_PUBLIC_SENTRY_URL: process.env.NEXT_PUBLIC_SENTRY_URL,
     NEXT_PUBLIC_SENTRY_ORG: process.env.NEXT_PUBLIC_SENTRY_ORG,