fix: clean stale auth state on login/setup, unify language switcher

- LoginPage/SetupPage: clear localStorage auth tokens on mount
- AuthContext: clear onboarding state on register, invalidate config on logout
- Extract shared LanguageSwitcher component for consistent UI
- Merge duplicate config import in AuthContext

web/src/components/auth/LoginPage.tsx

@@ -5,6 +5,7 @@ import { useAuth } from '../../contexts/AuthContext'
 import { useLanguage } from '../../contexts/LanguageContext'
 import { t } from '../../i18n/translations'
 import { DeepVoidBackground } from '../common/DeepVoidBackground'
+import { LanguageSwitcher } from '../common/LanguageSwitcher'
 import { OnboardingModeSelector } from './OnboardingModeSelector'
 import type { UserMode } from '../../lib/onboarding'
 
@@ -19,6 +20,14 @@ export function LoginPage() {
   const [expiredToastId, setExpiredToastId] = useState<string | number | null>(null)
   const [mode, setMode] = useState<UserMode>('beginner')
 
+  // Clean up stale auth state once on mount
+  useEffect(() => {
+    localStorage.removeItem('auth_token')
+    localStorage.removeItem('auth_user')
+    localStorage.removeItem('user_id')
+  }, [])
+
+  // Show session-expired toast (re-runs on language change to update text)
   useEffect(() => {
     if (sessionStorage.getItem('from401') === 'true') {
       const id = toast.warning(t('sessionExpired', language), { duration: Infinity })
@@ -44,6 +53,8 @@ export function LoginPage() {
 
   return (
     <DeepVoidBackground disableAnimation>
+      <LanguageSwitcher />
+
       <div className="flex-1 flex items-center justify-center px-4 py-16">
         <div className="w-full max-w-sm">