Add internal authentication configuration and middleware support

- Enable internal authentication via environment variable in .env.example
- Update nuxt.config.ts to include internalAuthEnabled check
- Modify internal-auth middleware to conditionally enforce authentication based on the new configuration

.env.example

@@ -112,6 +112,7 @@ SMTP_FROM=noreply@experimenta.science
 # INTERNAL PAGES (Styleguide, Admin)
 # ==============================================
 # HTTP Basic Auth credentials for /internal/* routes
+INTERNAL_AUTH_ENABLED=true
 INTERNAL_AUTH_USERNAME=experimenta
 INTERNAL_AUTH_PASSWORD=change-me-to-secure-password