feat: replace histoire with a minimal in-house stories harness

Histoire is unmaintained and blocks upgrading Vite and other dev
dependencies. Replace it with a small standalone Vite app that keeps the
same `<Story>`/`<Variant>` template API so story files need no changes.

- Add vite.stories.config.ts and app/javascript/stories/ (sidebar tree
  with collapsible groups, search, dark-mode and RTL/LTR toggles, and a
  per-story error boundary)
- Globally register Story/Variant; migrate the histoire setup (store,
  i18n, plugins/directives) and seed a memory router for route-aware
  components
- Repoint story:dev/build/preview scripts and drop the histoire deps
- Remove histoire.config.ts, histoire.setup.ts and histoire.scss

app/javascript/stories/setup.js

@@ -0,0 +1,73 @@
+import '../design-system/stories.scss';
+import { createI18n } from 'vue-i18n';
+import { createRouter, createMemoryHistory } from 'vue-router';
+import { vResizeObserver } from '@vueuse/components';
+import FloatingVue from 'floating-vue';
+import VueDOMPurifyHTML from 'vue-dompurify-html';
+import { directive as onClickaway } from 'vue3-click-away';
+import dashboardI18n from 'dashboard/i18n';
+import widgetI18n from 'widget/i18n';
+import store from 'dashboard/store';
+import { domPurifyConfig } from 'shared/helpers/HTMLSanitizer.js';
+
+// A throwaway router so composables like useRoute()/useAccount() resolve a real
+// route object instead of throwing. We seed realistic Help Center params
+// (accountId/portalSlug/locale) so page-level stories that key off the route
+// render instead of bailing out. Story selection itself is hash-based and
+// independent of this router.
+const noop = { render: () => null };
+const router = createRouter({
+  history: createMemoryHistory(),
+  routes: [
+    {
+      path: '/app/accounts/:accountId/portals/:portalSlug/:locale?',
+      name: 'stories',
+      component: noop,
+    },
+    { path: '/:pathMatch(.*)*', name: 'fallback', component: noop },
+  ],
+});
+router.replace('/app/accounts/1/portals/chatwoot/en');
+
+function mergeMessages(...sources) {
+  return sources.reduce((acc, src) => {
+    Object.keys(src).forEach(key => {
+      if (
+        acc[key] &&
+        typeof acc[key] === 'object' &&
+        typeof src[key] === 'object'
+      ) {
+        acc[key] = mergeMessages(acc[key], src[key]);
+      } else {
+        acc[key] = src[key];
+      }
+    });
+    return acc;
+  }, {});
+}
+
+const i18n = createI18n({
+  legacy: false, // https://github.com/intlify/vue-i18n/issues/1902
+  locale: 'en',
+  messages: mergeMessages(
+    structuredClone(dashboardI18n),
+    structuredClone(widgetI18n)
+  ),
+});
+
+// Registers the same global plugins/directives the dashboard components expect
+// at runtime, so stories render identically to the real app.
+export function setupApp(app) {
+  app.use(store);
+  app.use(i18n);
+  app.use(router);
+  app.use(FloatingVue, {
+    instantMove: true,
+    arrowOverflow: false,
+    disposeTimeout: 5000000,
+  });
+
+  app.directive('resize', vResizeObserver);
+  app.use(VueDOMPurifyHTML, domPurifyConfig);
+  app.directive('on-clickaway', onClickaway);
+}