fix(web): withPayload + redirect admin + externalized packages

Final working pattern: withPayload via require() for build
compatibility, admin page replaced with redirect (no RootPage
import = no React #130), payload packages externalized from
turbopack bundle. Blog/changelog use server-side getPayload().

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

apps/web/src/app/(payload)/payload/importMap.js

@@ -0,0 +1 @@
+export const importMap = {};