Add internal styleguide page with HTTP Basic Auth

- Create /internal/styleguide route with protected layout
- Add styleguide.vue layout with auth protection
- Configure internal pages auth credentials in .env.example
- Update nuxt.config for internal routes handling

.env.example

@@ -108,6 +108,13 @@ SMTP_FROM=noreply@experimenta.science
 # SENTRY_DSN=https://xxx@sentry.io/xxx
 # SENTRY_ENVIRONMENT=development
 
+# ==============================================
+# INTERNAL PAGES (Styleguide, Admin)
+# ==============================================
+# HTTP Basic Auth credentials for /internal/* routes
+INTERNAL_AUTH_USERNAME=experimenta
+INTERNAL_AUTH_PASSWORD=change-me-to-secure-password
+
 # ==============================================
 # DEVELOPMENT TOOLS
 # ==============================================