Load only specified flatpickr locale

Dynamically import only the requested flatpickr locale.

English locale is bundled by default, so passing null triggers flatpickr's built-in English fallback without an explicit import.

spec/javascripts/stimulus/flatpickr_controller_test.js

@@ -0,0 +1,43 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import { Application } from "stimulus";
+import FlatpickrController from "../../../app/webpacker/controllers/flatpickr_controller.js";
+
+describe("FlatpickrController", () => {
+  beforeAll(() => {
+    const application = Application.start();
+    application.register("flatpickr", FlatpickrController);
+  });
+
+  describe("#importFlatpickrLocale", () => {
+    describe("returns null to trigger flatpickr fallback to english", () => {
+      test.each([
+        ["when no base_locale is set", {}],
+        ["when base_locale doesn't match a Flatpickr locale", { base_locale: "invalid-locale" }],
+        ["when base_locale is 'en'", { base_locale: "en" }],
+      ])("%s", async (_description, i18nData) => {
+        const I18n = i18nData;
+        const controller = new FlatpickrController();
+        const locale = await controller.importFlatpickrLocale(I18n.base_locale);
+        expect(locale).toBeNull();
+      });
+    });
+
+    it("returns locale object for a supported locale (fr)", async () => {
+      const controller = new FlatpickrController();
+      const locale = await controller.importFlatpickrLocale("fr");
+      expect(locale).toBeInstanceOf(Object);
+      expect(locale).toHaveProperty("weekAbbreviation");
+      expect(locale.weekAbbreviation).toBe("Sem");
+    });
+
+    it("caches the locale object for repeated calls", async () => {
+      const controller = new FlatpickrController();
+      const locale1 = await controller.importFlatpickrLocale("fr");
+      const locale2 = await controller.importFlatpickrLocale("fr");
+      expect(locale1).toBe(locale2);
+    });
+  });
+});