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/i18n/TranslationContext.tsx

@@ -59,8 +59,10 @@ export function detectBrowserLanguage(): string | null {
     const exactMatch = supported.find(s => s.toLowerCase() === lang.toLowerCase())
     if (exactMatch) return exactMatch
 
-    // Portuguese variants → our code is 'br' (pt-BR)
-    if (lang.toLowerCase().startsWith('pt')) return 'br'
+    // pt-BR has no exact match (our code is 'br', not 'pt-BR'), so map it explicitly.
+    // pt-PT and bare 'pt' are NOT mapped — they fall through to null and let the
+    // server default or 'en' fallback apply instead.
+    if (lang.toLowerCase() === 'pt-br') return 'br'
 
     // Prefix match (e.g. 'de-AT' → 'de', 'zh-CN' → 'zh') — case-insensitive
     const prefix = lang.split('-')[0].toLowerCase()