fix(login): address review feedback on language dropdown PR

- Fix import path: use i18n barrel instead of TranslationContext directly
- Encapsulate localStorage key behind hasStoredLanguage() helper in settingsStore
- Fix pt-BR detection: only map pt-BR to br, pt-PT now returns null correctly
- Add comment linking server SUPPORTED_LANG_CODES to canonical client source
- Extract /api/config inline handler to routes/publicConfig.ts
- Add aria-haspopup, aria-expanded, role=listbox/option, aria-selected to dropdown
- Add 8 tests for detectBrowserLanguage (FE-COMP-I18N-016–023)
- Add 3 tests for setLanguageTransient (FE-STORE-SETTINGS-015–017)

server/src/routes/publicConfig.ts

@@ -0,0 +1,10 @@
+import express, { Request, Response } from 'express';
+import { DEFAULT_LANGUAGE } from '../config';
+
+const router = express.Router();
+
+router.get('/', (_req: Request, res: Response) => {
+  res.json({ defaultLanguage: DEFAULT_LANGUAGE });
+});
+
+export default router;