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/locales/index.ts

@@ -0,0 +1,12 @@
+import en, { type TranslationKey } from "./en"
+import zh from "./zh"
+
+export type { TranslationKey }
+export type SupportedLocale = "en" | "zh"
+export type LocaleMessages = Record<TranslationKey, string>
+
+type LocaleMap = Record<SupportedLocale, LocaleMessages>
+export const locales: LocaleMap = {
+  en,
+  zh,
+}