fix: correct portal links to use environment-specific URLs

The portal homepage was generating incorrect links to standards sites
because customFields.docsEnv was missing, causing the HomepageFeatures
component to default to production URLs.

- Add docsEnv to portal's customFields in docusaurus.config.ts
- Portal now correctly generates localhost URLs for local development
- Standards links now point to correct environment-specific URLs

This resolves the issue where portal links pointed to production URLs
regardless of the current environment.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: jonphipps

portal/docusaurus.config.ts

@@ -64,6 +64,8 @@ const config: Config = deepmerge(
     customFields: {
       // Current environment for client-side components
       environment,
+      // Environment for site URL generation
+      docsEnv: currentEnv,
       // Portal-specific vocabulary configuration (minimal since portal doesn't have RDF content)
       vocabularyDefaults: {
         prefix: "ifla",