Add i18n system with 4-language support (en/zh/ja/ko)

- Add I18nContext provider with useI18n hook and localStorage persistence
- Add translations.ts with complete strings for English, Chinese, Japanese, Korean
- Wrap App in I18nProvider, replace hardcoded toast messages with t() calls
- Add language selector to SettingsModal with Globe icon
- Add localization credits in About section
- Default language: English
- Add I18N_USAGE.md documentation

I18N_USAGE.md

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+# ACE-Step UI — Internationalization Guide
+
+## Overview
+
+The project supports 4 languages: English (default), Chinese, Japanese, and Korean.
+
+## Architecture
+
+```
+ace-step-ui/
+├── i18n/
+│   └── translations.ts          # All translation strings
+├── context/
+│   └── I18nContext.tsx          # React context + useI18n hook
+└── components/                   # i18n-enabled components
+```
+
+## Usage
+
+### 1. Use translations in a component
+
+```tsx
+import { useI18n } from '../context/I18nContext';
+
+function YourComponent() {
+  const { t } = useI18n();
+  return <div>{t('yourTranslationKey')}</div>;
+}
+```
+
+### 2. Switch language
+
+Users can switch language in Settings. Programmatically:
+
+```tsx
+const { language, setLanguage } = useI18n();
+setLanguage('en'); // 'en' | 'zh' | 'ja' | 'ko'
+```
+
+### 3. Add a new translation key
+
+Add the key to all 4 languages in `i18n/translations.ts`:
+
+```typescript
+export const translations = {
+  en: { yourNewKey: 'English text' },
+  zh: { yourNewKey: '中文文本' },
+  ja: { yourNewKey: '日本語テキスト' },
+  ko: { yourNewKey: '한국어 텍스트' },
+};
+```
+
+## Language Persistence
+
+The selected language is stored in `localStorage` and restored on next visit. Default is English.
+
+## Notes
+
+- All keys must exist in every language object
+- TypeScript's `TranslationKey` type enforces key safety
+- If a key is missing, the raw key name is returned as fallback