fix(web): enable turbopack + payload by unbundling richtext-lexical

The CSS import error was caused by richtext-lexical being in
serverExternalPackages — Node can't require .css files. Removing
it lets Turbopack bundle it (handling CSS natively). Other payload
packages stay external (they don't import CSS).

Restores turbopack as the default production bundler.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/next.config.ts

@@ -87,7 +87,6 @@ const config: NextConfig = {
     "payload",
     "@payloadcms/db-postgres",
     "@payloadcms/db-sqlite",
-    "@payloadcms/richtext-lexical",
     "sharp",
   ],
   turbopack: {