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)

client/src/store/settingsStore.ts

@@ -15,6 +15,11 @@ interface SettingsState {
   updateSettings: (settingsObj: Partial<Settings>) => Promise<void>
 }
 
+// Returns true when the user has explicitly chosen a language (persisted in localStorage).
+// Use this instead of reading localStorage directly so the key stays encapsulated here.
+export const hasStoredLanguage = (): boolean =>
+  typeof localStorage !== 'undefined' && !!localStorage.getItem('app_language')
+
 export const useSettingsStore = create<SettingsState>((set, get) => ({
   settings: {
     map_tile_url: '',