Expose sortBy in loadConfig allowlist

`loadConfig` builds its output from an explicit allowlist, so the new
`sortBy` key from `config.default.yaml` was silently dropped. That made
`siteMetadata.sortBy` undefined at build time, which in turn caused
Gatsby's schema inference to omit the field — and the GraphQL query
in `useConfigAndConceptSchemes` failed CI with
`Cannot query field "sortBy" on type "SiteSiteMetadata"`.

Add `sortBy` to the allowlist (default "prefLabel") and update the
config snapshot test.

Author: sroertgen

src/common.js

@@ -144,6 +144,7 @@ function loadConfig(configFile, defaultFile) {
       userConfig.searchableAttributes || defaults.searchableAttributes,
     customDomain: userConfig.custom_domain || "",
     failOnValidation: userConfig.fail_on_validation,
+    sortBy: userConfig.sortBy || defaults.sortBy || "prefLabel",
   }
 
   // check if all relevant colors are contained, otherwise use default colors

test/config.test.js

@@ -27,6 +27,7 @@ describe("Config Parsing", () => {
       },
       customDomain: "",
       failOnValidation: true,
+      sortBy: "prefLabel",
       fonts: {
         regular: {
           font_family: "Aladin",