fix(web): payload at /payload route (cuidecar pattern)

Replicate working cuidecar Payload setup:
- require() instead of ESM import for withPayload
- routes.admin = "/payload" to avoid /admin conflicts
- (payload)/payload/ route group with own layout + importMap

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

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

@@ -0,0 +1,2 @@
+// Auto-generated by Payload at build time. Start empty.
+export const importMap = {};