feat(i18n): add localization infrastructure and English catalog

Refs #17

src/i18n/locales.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+import {
+  APP_LOCALES,
+  DEFAULT_LOCALE,
+  isAppLocale,
+  localeDirection,
+  resolveAppLocale,
+} from './locales';
+
+describe('app locale metadata', () => {
+  it('keeps the planned locale set stable', () => {
+    expect(APP_LOCALES).toEqual(['en', 'zh-CN', 'he', 'fa', 'uk']);
+    expect(DEFAULT_LOCALE).toBe('en');
+  });
+
+  it('normalizes common system language tags', () => {
+    expect(resolveAppLocale('en-US')).toBe('en');
+    expect(resolveAppLocale('zh-Hans-CN')).toBe('zh-CN');
+    expect(resolveAppLocale('iw-IL')).toBe('he');
+    expect(resolveAppLocale('fa-IR')).toBe('fa');
+    expect(resolveAppLocale('uk-UA')).toBe('uk');
+    expect(resolveAppLocale('de-DE')).toBe('en');
+  });
+
+  it('marks the planned RTL locales correctly', () => {
+    expect(localeDirection('en')).toBe('ltr');
+    expect(localeDirection('zh-CN')).toBe('ltr');
+    expect(localeDirection('he')).toBe('rtl');
+    expect(localeDirection('fa')).toBe('rtl');
+    expect(localeDirection('uk')).toBe('ltr');
+  });
+
+  it('guards locale values before they reach i18next', () => {
+    expect(isAppLocale('en')).toBe(true);
+    expect(isAppLocale('fa')).toBe(true);
+    expect(isAppLocale('de')).toBe(false);
+    expect(isAppLocale(null)).toBe(false);
+  });
+});