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.

packages/ui/src/index.tsx

@@ -1,20 +1,8 @@
-import { cx } from 'class-variance-authority';
-import { twMerge } from 'tailwind-merge';
-
-export const cn = (...inputs: Parameters<typeof cx>) => twMerge(cx(inputs));
-
-export const ccn = ({
-  context,
-  className,
-  on = '',
-  off = '',
-}: {
-  context: boolean;
-  className: string;
-  on: string;
-  off: string;
-}) => twMerge(className, context ? on : off);
-
+// Re-exports only. The Next app lists @spoon/ui in `optimizePackageImports`,
+// which resolves each named import straight to its own module so a page that
+// uses Button never loads Chart (and Recharts). That only works while nothing
+// is declared in this file.
+export { cn, ccn } from './utils';
 export {
   Accordion,
   AccordionItem,

packages/ui/src/utils.ts

@@ -0,0 +1,16 @@
+import { cx } from 'class-variance-authority';
+import { twMerge } from 'tailwind-merge';
+
+export const cn = (...inputs: Parameters<typeof cx>) => twMerge(cx(inputs));
+
+export const ccn = ({
+  context,
+  className,
+  on = '',
+  off = '',
+}: {
+  context: boolean;
+  className: string;
+  on: string;
+  off: string;
+}) => twMerge(className, context ? on : off);