feat(i18n): add localization infrastructure and English catalog

Refs #17

src/i18n/locales.ts

@@ -0,0 +1,41 @@
+export const APP_LOCALES = ['en', 'zh-CN', 'he', 'fa', 'uk'] as const;
+
+export type AppLocale = typeof APP_LOCALES[number];
+export type AppLocaleDirection = 'ltr' | 'rtl';
+
+export const DEFAULT_LOCALE: AppLocale = 'en';
+
+export const APP_LOCALE_METADATA: Record<AppLocale, {
+  direction: AppLocaleDirection;
+  isTranslated: boolean;
+}> = {
+  en: { direction: 'ltr', isTranslated: true },
+  'zh-CN': { direction: 'ltr', isTranslated: false },
+  he: { direction: 'rtl', isTranslated: false },
+  fa: { direction: 'rtl', isTranslated: false },
+  uk: { direction: 'ltr', isTranslated: false },
+};
+
+export const isAppLocale = (value: unknown): value is AppLocale =>
+  typeof value === 'string' && (APP_LOCALES as readonly string[]).includes(value);
+
+/**
+ * Resolve a system or persisted BCP 47 language tag to a planned Firelink
+ * locale. Non-English locales intentionally fall back to English resources
+ * until their translations are added.
+ */
+export const resolveAppLocale = (value: string | null | undefined): AppLocale => {
+  const normalized = value?.trim().replace(/_/g, '-').toLowerCase();
+  if (!normalized) return DEFAULT_LOCALE;
+
+  if (normalized === 'zh-cn' || normalized.startsWith('zh-')) return 'zh-CN';
+  if (normalized === 'he' || normalized.startsWith('he-') || normalized === 'iw' || normalized.startsWith('iw-')) return 'he';
+  if (normalized === 'fa' || normalized.startsWith('fa-')) return 'fa';
+  if (normalized === 'uk' || normalized.startsWith('uk-')) return 'uk';
+  if (normalized === 'en' || normalized.startsWith('en-')) return 'en';
+
+  return DEFAULT_LOCALE;
+};
+
+export const localeDirection = (locale: AppLocale): AppLocaleDirection =>
+  APP_LOCALE_METADATA[locale].direction;