FIX: Use correct subfolder format for ember-cli config (#23754)

Ember expects a trailing slash on this value, which is different to the Rails app's behavior. Values without a trailing slash seemed to work for legacy ember-cli builds, but would lead to errors under embroider.

app/assets/javascripts/discourse/config/environment.js

@@ -4,7 +4,7 @@ module.exports = function (environment) {
   const ENV = {
     modulePrefix: "discourse",
     environment,
-    rootURL: process.env.DISCOURSE_RELATIVE_URL_ROOT || "/",
+    rootURL: `${process.env.DISCOURSE_RELATIVE_URL_ROOT ?? ""}/`, // Add a trailing slash (not required by the Rails app in this env variable)
     locationType: "history",
     historySupportMiddleware: false,
     EmberENV: {