fix: resolve leaflet markercluster ESM build failure under pnpm

Use esmExternals loose and transpile @changey/react-leaflet-markercluster so its CJS dist can require @react-leaflet/core during Next.js production builds.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: mrTuomoK

next.base.config.mjs

@@ -74,7 +74,11 @@ if (NEXTJS_SENTRY_DEBUG) {
 
 // Tell webpack to compile those packages
 // @link https://www.npmjs.com/package/next-transpile-modules
-const tmModules = ['@events-helsinki/components'];
+const tmModules = [
+  '@events-helsinki/components',
+  // CJS dist requires @react-leaflet/core; transpile for Next esmExternals
+  '@changey/react-leaflet-markercluster',
+];
 
 const projectFolder = process.cwd();
 
@@ -134,8 +138,8 @@ const nextBaseConfig = ({
     outputFileTracingRoot: __dirname,
 
     experimental: {
-      // Prefer loading of ES Modules over CommonJS
-      esmExternals: true,
+      // 'loose' allows CJS deps (e.g. @changey/react-leaflet-markercluster) to require ESM packages
+      esmExternals: 'loose',
       // Experimental monorepo support
       externalDir: true,
       scrollRestoration: true,