Support translations in Storybook: locale toolbar (#6843)

## What

Adds a globe toolbar to Storybook so any story can be previewed in all
42 supported languages (the i18n init already on `main` was
English-only).

## How

Bundles every locale's `translation.toml` via a `?raw` glob into i18next
resources, and switches language on toolbar change. RTL locales (`ar`,
`fa`) flip `document.dir`.

---------

Co-authored-by: James Brunton <jbrunton96@gmail.com>

frontend/.storybook/preview.tsx

@@ -1,3 +1,4 @@
+/// <reference types="vite/client" />
 // Storybook compiles .storybook/* with the classic JSX runtime, so the JSX in
 // the decorators below transpiles to React.createElement and needs React in
 // scope. (The app + story files use the automatic runtime via the portal vite
@@ -22,28 +23,38 @@ import { configureSupabase } from "@proprietary/auth/supabase/supabaseClient";
 import i18next from "i18next";
 import { initReactI18next } from "react-i18next";
 import { parse as parseToml } from "smol-toml";
-// Load the real English copy so stories render human text, not raw keys. Bundled
-// synchronously via ?raw so it's present on the very first render (no async flash).
-import enTranslationToml from "@public/locales/en-US/translation.toml?raw";
+import { rtlLanguages, supportedLanguages } from "@core/i18n/languages";
 
 import "@mantine/core/styles.css";
 import "@core/tokens/tokens.css";
 import "@core/theme/index.css";
 import "@core/tokens/base.css";
 
-// Storybook-only: init react-i18next with the real English resources parsed from
-// the app's TOML, so t(key) renders the shipped copy (e.g. "No sources connected
-// yet") rather than the raw key. Falls back to an empty bundle if parsing ever
-// fails, so a malformed TOML can't take the whole Storybook down.
-function parseEnTranslation(): Record<string, unknown> {
+// Storybook-only: bundle every shipped locale's TOML at build time via a ?raw
+// glob, so the toolbar language switcher can flip between all languages with no
+// async fetch (Storybook has no backend to serve /locales/). t(key) then renders
+// the shipped copy (e.g. "No sources connected yet") rather than the raw key.
+const localeModules = import.meta.glob<string>(
+  "../editor/public/locales/*/translation.toml",
+  { query: "?raw", import: "default", eager: true },
+);
+
+// Parse each locale into an i18next resources map. A malformed TOML degrades to
+// an empty bundle for that one locale (its keys fall back to en-US) rather than
+// taking the whole Storybook down.
+const resources: Record<string, { translation: Record<string, unknown> }> = {};
+for (const [path, raw] of Object.entries(localeModules)) {
+  const lng = path.match(/\/locales\/([^/]+)\/translation\.toml$/)?.[1];
+  if (!lng) continue;
+  let translation: Record<string, unknown>;
   try {
-    return parseToml(enTranslationToml) as Record<string, unknown>;
+    translation = parseToml(raw) as Record<string, unknown>;
   } catch {
-    return {};
+    translation = {};
   }
+  resources[lng] = { translation };
 }
 
-const enTranslationResources = parseEnTranslation();
 if (!i18next.isInitialized) {
   // initImmediate: false → initialise synchronously from the inline resources
   // (there's no async backend here), so i18next is ready before the first story
@@ -51,24 +62,25 @@ if (!i18next.isInitialized) {
   void i18next.use(initReactI18next).init({
     lng: "en-US",
     fallbackLng: "en-US",
-    supportedLngs: ["en-US"],
-    resources: { "en-US": { translation: enTranslationResources } },
+    supportedLngs: Object.keys(resources),
+    resources,
     interpolation: { escapeValue: false },
     react: { useSuspense: false },
     initImmediate: false,
   });
 } else {
   // Something initialised i18next first (e.g. the app's async TOML backend):
-  // inject the shipped English copy under the resolved language so t() renders
-  // real copy, not raw keys.
-  const lng = i18next.resolvedLanguage || i18next.language || "en-US";
-  i18next.addResourceBundle(
-    lng,
-    "translation",
-    enTranslationResources,
-    true,
-    true,
-  );
+  // inject every shipped locale's copy so t() renders real copy, not raw keys,
+  // and the toolbar switcher can still change to any of them.
+  for (const [lng, bundle] of Object.entries(resources)) {
+    i18next.addResourceBundle(
+      lng,
+      "translation",
+      bundle.translation,
+      true,
+      true,
+    );
+  }
 }
 
 // Start MSW once. Storybook runs in a browser so this uses the service worker.
@@ -156,6 +168,19 @@ function SchemeSetup({ scheme }: { scheme: "light" | "dark" }) {
   return null;
 }
 
+/** Switches i18next to the toolbar locale and keeps document dir/lang in sync. */
+const withLocale: Decorator = (Story, context) => {
+  const locale = (context.globals.locale as string) ?? "en-US";
+  useEffect(() => {
+    void i18next.changeLanguage(locale);
+    document.documentElement.dir = rtlLanguages.includes(locale)
+      ? "rtl"
+      : "ltr";
+    document.documentElement.lang = locale;
+  }, [locale]);
+  return <Story />;
+};
+
 const withProviders: Decorator = (Story, context) => {
   const tier = (context.globals.tier as Tier) ?? "pro";
   const linkState =
@@ -243,8 +268,22 @@ const preview: Preview = {
         dynamicTitle: true,
       },
     },
+    locale: {
+      name: "Locale",
+      description: "Active language — drives useTranslation() in all stories",
+      defaultValue: "en-US",
+      toolbar: {
+        icon: "globe",
+        items: Object.entries(supportedLanguages).map(([value, title]) => ({
+          value,
+          title: `${value} - ${title}`,
+        })),
+        dynamicTitle: true,
+      },
+    },
   },
   decorators: [
+    withLocale,
     withProviders,
     withThemeByDataAttribute({
       themes: { light: "light", dark: "dark" },