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/config.ts

@@ -102,6 +102,8 @@ export const ENCRYPTION_KEY = _encryptionKey;
 // DEFAULT_LANGUAGE sets the language shown on the login page before the user
 // selects one. Only applies when the user has no saved language preference.
 // Supported values: de, en, es, fr, hu, nl, br, cs, pl, ru, zh, zh-TW, it, ar
+// Must stay in sync with client/src/i18n/supportedLanguages.ts (canonical source).
+// Kept duplicated here because server and client are separate npm packages.
 const SUPPORTED_LANG_CODES = ['de', 'en', 'es', 'fr', 'hu', 'nl', 'br', 'cs', 'pl', 'ru', 'zh', 'zh-TW', 'it', 'ar'];
 const rawDefaultLang = process.env.DEFAULT_LANGUAGE || 'en';
 if (!SUPPORTED_LANG_CODES.includes(rawDefaultLang)) {