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/common/LanguageSwitcher.tsx

@@ -0,0 +1,34 @@
+import React from 'react'
+import { Globe } from 'lucide-react'
+import { useLanguage } from '../../contexts/LanguageContext'
+import type { Language } from '../../i18n/translations'
+
+const languages: { code: Language; label: string }[] = [
+  { code: 'zh', label: '中文' },
+  { code: 'en', label: 'EN' },
+  { code: 'id', label: 'ID' },
+]
+
+export function LanguageSwitcher() {
+  const { language, setLanguage } = useLanguage()
+
+  return (
+    <div className="absolute top-4 right-4 z-50 flex items-center gap-1 rounded-lg p-1 border border-white/10 bg-white/5 backdrop-blur-sm">
+      <Globe size={14} className="text-zinc-500 ml-1.5 mr-0.5" />
+      {languages.map(({ code, label }) => (
+        <button
+          key={code}
+          type="button"
+          onClick={() => setLanguage(code)}
+          className={`px-2.5 py-1 rounded text-xs font-semibold transition-all ${
+            language === code
+              ? 'bg-nofx-gold/15 text-nofx-gold'
+              : 'text-zinc-500 hover:text-zinc-300 bg-transparent'
+          }`}
+        >
+          {label}
+        </button>
+      ))}
+    </div>
+  )
+}