feat(i18n): add toast i18n with en/zh locale and plugin config support

- Add src/locales/ with en baseline and zh overrides (Partial<Record> fallback)
- Add src/shared/i18n.ts with initI18n/t/setLocale/getLocale (LANG env auto-detect)
- Add I18nConfigSchema with locale field to plugin config
- Internationalize 13 hardcoded strings in task-toast-manager
- Add 18 unit tests for i18n module
- Pin manager tests to en locale for determinism

src/shared/i18n.ts

@@ -0,0 +1,51 @@
+import { type SupportedLocale, type TranslationKey, locales } from "../locales"
+
+let currentLang: SupportedLocale = "en"
+let fallbackLang: SupportedLocale = "en"
+
+function isSupportedLocale(locale: string): locale is SupportedLocale {
+  return locale in locales
+}
+
+function isTranslationKey(key: string): key is TranslationKey {
+  return key in locales.en
+}
+
+function detectLocale(): SupportedLocale {
+  const envLang = process.env.LANG ?? ""
+  const lang = envLang.split(".")[0]?.split("_")[0]?.toLowerCase() ?? "en"
+  const supported: Record<string, SupportedLocale> = { zh: "zh" }
+  return supported[lang] ?? "en"
+}
+
+export function initI18n(opts?: { locale?: string; fallback?: string }): void {
+  currentLang = opts?.locale && isSupportedLocale(opts.locale)
+    ? opts.locale
+    : detectLocale()
+  fallbackLang = opts?.fallback && isSupportedLocale(opts.fallback)
+    ? opts.fallback
+    : "en"
+  if (!isSupportedLocale(currentLang)) currentLang = "en"
+}
+
+export function getLocale(): SupportedLocale {
+  return currentLang
+}
+
+export function setLocale(lang: string): void {
+  if (isSupportedLocale(lang)) currentLang = lang
+}
+
+export function t(key: TranslationKey, params?: Record<string, string | number>): string
+export function t(key: string, params?: Record<string, string | number>): string
+export function t(key: string, params?: Record<string, string | number>): string {
+  let msg = key
+  if (isTranslationKey(key)) {
+    msg = locales[currentLang][key] ?? locales[fallbackLang][key] ?? key
+  }
+  if (!params) return msg
+  return msg.replace(/\{\{(\w+)\}\}/g, (_match: string, name: string) => {
+    const value = params[name]
+    return value != null ? String(value) : `{{${name}}}`
+  })
+}