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

context/I18nContext.tsx

@@ -0,0 +1,40 @@
+import React, { createContext, useContext, useState, ReactNode } from 'react';
+import { translations, Language, TranslationKey } from '../i18n/translations';
+
+interface I18nContextType {
+  language: Language;
+  setLanguage: (lang: Language) => void;
+  t: (key: TranslationKey) => string;
+}
+
+const I18nContext = createContext<I18nContextType | undefined>(undefined);
+
+export const I18nProvider: React.FC<{ children: ReactNode }> = ({ children }) => {
+  const [language, setLanguage] = useState<Language>(() => {
+    const stored = localStorage.getItem('language') as Language;
+    return stored === 'zh' || stored === 'en' || stored === 'ja' || stored === 'ko' ? stored : 'en';
+  });
+
+  const handleSetLanguage = (lang: Language) => {
+    setLanguage(lang);
+    localStorage.setItem('language', lang);
+  };
+
+  const t = (key: TranslationKey): string => {
+    return translations[language][key] || key;
+  };
+
+  return (
+    <I18nContext.Provider value={{ language, setLanguage: handleSetLanguage, t }}>
+      {children}
+    </I18nContext.Provider>
+  );
+};
+
+export const useI18n = () => {
+  const context = useContext(I18nContext);
+  if (!context) {
+    throw new Error('useI18n must be used within I18nProvider');
+  }
+  return context;
+};