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/tests/unit/i18n/index.test.ts

@@ -8,6 +8,7 @@ import {
   getIntlLanguage,
   isRtlLanguage,
   SUPPORTED_LANGUAGES,
+  detectBrowserLanguage,
 } from '../../../src/i18n'
 import { resetAllStores, seedStore } from '../../helpers/store'
 import { useSettingsStore } from '../../../src/store/settingsStore'
@@ -96,6 +97,56 @@ describe('SUPPORTED_LANGUAGES', () => {
   })
 })
 
+// ── FE-COMP-I18N-016 to 023: detectBrowserLanguage ───────────────────────────
+
+describe('detectBrowserLanguage', () => {
+  afterEach(() => {
+    Object.defineProperty(navigator, 'languages', { value: [], configurable: true })
+    Object.defineProperty(navigator, 'language', { value: '', configurable: true })
+  })
+
+  it('FE-COMP-I18N-016: exact match returns the matched code', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['de'], configurable: true })
+    expect(detectBrowserLanguage()).toBe('de')
+  })
+
+  it('FE-COMP-I18N-017: region-tagged exact match (zh-TW) returns zh-TW', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['zh-TW'], configurable: true })
+    expect(detectBrowserLanguage()).toBe('zh-TW')
+  })
+
+  it('FE-COMP-I18N-018: prefix match (de-AT → de)', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['de-AT'], configurable: true })
+    expect(detectBrowserLanguage()).toBe('de')
+  })
+
+  it('FE-COMP-I18N-019: pt-PT returns null (European Portuguese is a distinct language)', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['pt-PT'], configurable: true })
+    expect(detectBrowserLanguage()).toBeNull()
+  })
+
+  it('FE-COMP-I18N-020: pt-BR maps to br', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['pt-BR'], configurable: true })
+    expect(detectBrowserLanguage()).toBe('br')
+  })
+
+  it('FE-COMP-I18N-021: first-match-wins across multiple entries', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['xx-XX', 'fr'], configurable: true })
+    expect(detectBrowserLanguage()).toBe('fr')
+  })
+
+  it('FE-COMP-I18N-022: unknown language returns null', () => {
+    Object.defineProperty(navigator, 'languages', { value: ['xx'], configurable: true })
+    expect(detectBrowserLanguage()).toBeNull()
+  })
+
+  it('FE-COMP-I18N-023: falls back to navigator.language when navigator.languages is empty', () => {
+    Object.defineProperty(navigator, 'languages', { value: [], configurable: true })
+    Object.defineProperty(navigator, 'language', { value: 'es', configurable: true })
+    expect(detectBrowserLanguage()).toBe('es')
+  })
+})
+
 // ── FE-COMP-I18N-010 to 015: TranslationProvider + useTranslation ─────────────
 
 describe('TranslationProvider + useTranslation integration', () => {

client/tests/unit/stores/settingsStore.test.ts

@@ -170,6 +170,37 @@ describe('settingsStore', () => {
     });
   });
 
+  describe('FE-STORE-SETTINGS-015: setLanguageTransient updates state without touching localStorage', () => {
+    it('sets language in state but does not write to localStorage', () => {
+      localStorage.clear();
+
+      useSettingsStore.getState().setLanguageTransient('fr');
+
+      expect(useSettingsStore.getState().settings.language).toBe('fr');
+      expect(localStorage.getItem('app_language')).toBeNull();
+    });
+  });
+
+  describe('FE-STORE-SETTINGS-016: setLanguageTransient rejects unsupported language code', () => {
+    it('leaves state unchanged for an unknown code', () => {
+      const before = useSettingsStore.getState().settings.language;
+
+      useSettingsStore.getState().setLanguageTransient('xx');
+
+      expect(useSettingsStore.getState().settings.language).toBe(before);
+    });
+  });
+
+  describe('FE-STORE-SETTINGS-017: setLanguageTransient does not overwrite an explicit localStorage choice', () => {
+    it('localStorage remains unchanged after a transient set', () => {
+      localStorage.setItem('app_language', 'de');
+
+      useSettingsStore.getState().setLanguageTransient('es');
+
+      expect(localStorage.getItem('app_language')).toBe('de');
+    });
+  });
+
   describe('FE-STORE-SETTINGS-014: updateSetting API failure leaves optimistic state', () => {
     it('throws on API failure but keeps the optimistic state', async () => {
       server.use(