web: i18n support — English, 简体中文, 繁體中文, Italiano

Lightweight i18n without react-i18next: a LocaleProvider context +
useLocale() hook with {{var}} interpolation, auto-detect from
navigator.language, persisted to localStorage.

98 translation keys across 4 locale files (en/zh-CN/zh-TW/it).
All user-visible strings in App/Brain/Profiling/ErrorBoundary are
now t() calls. Language switcher added to the sidebar footer.

Build clean (tsc + vite), 18 tests pass.

web/src/i18n/index.ts

@@ -0,0 +1,78 @@
+import { createContext, useContext, useState, useCallback, useMemo, type ReactNode } from "react"
+import { createElement } from "react"
+import en from "./en"
+import zhCN from "./zh-CN"
+import zhTW from "./zh-TW"
+import it from "./it"
+
+const LOCALES = [
+  { code: "en", label: "English" },
+  { code: "zh-CN", label: "简体中文" },
+  { code: "zh-TW", label: "繁體中文" },
+  { code: "it", label: "Italiano" },
+] as const
+
+const DICTS: Record<string, Record<string, string>> = {
+  "en": en,
+  "zh-CN": zhCN,
+  "zh-TW": zhTW,
+  "it": it,
+}
+
+const STORAGE_KEY = "colibri-locale"
+
+function detectLocale(): string {
+  try {
+    const saved = localStorage.getItem(STORAGE_KEY)
+    if (saved && DICTS[saved]) return saved
+  } catch {}
+  const nav = navigator.language || ""
+  if (DICTS[nav]) return nav
+  const prefix = nav.split("-")[0]
+  if (prefix === "zh") return nav.includes("TW") || nav.includes("Hant") ? "zh-TW" : "zh-CN"
+  for (const { code } of LOCALES) if (code.startsWith(prefix)) return code
+  return "en"
+}
+
+function interpolate(template: string, vars?: Record<string, string | number>): string {
+  if (!vars) return template
+  return template.replace(/\{\{(\w+)\}\}/g, (_, key) => String(vars[key] ?? `{{${key}}}`))
+}
+
+interface LocaleContext {
+  locale: string
+  setLocale: (code: string) => void
+  t: (key: string, vars?: Record<string, string | number>) => string
+  locales: readonly { code: string; label: string }[]
+}
+
+const Ctx = createContext<LocaleContext>({
+  locale: "en",
+  setLocale: () => {},
+  t: (key) => key,
+  locales: LOCALES,
+})
+
+export function LocaleProvider({ children }: { children: ReactNode }) {
+  const [locale, setLocaleState] = useState(detectLocale)
+
+  const setLocale = useCallback((code: string) => {
+    if (!DICTS[code]) return
+    setLocaleState(code)
+    try { localStorage.setItem(STORAGE_KEY, code) } catch {}
+  }, [])
+
+  const t = useCallback((key: string, vars?: Record<string, string | number>) => {
+    const dict = DICTS[locale] || en
+    const template = dict[key] ?? en[key] ?? key
+    return interpolate(template, vars)
+  }, [locale])
+
+  const value = useMemo(() => ({ locale, setLocale, t, locales: LOCALES }), [locale, setLocale, t])
+
+  return createElement(Ctx.Provider, { value }, children)
+}
+
+export function useLocale() {
+  return useContext(Ctx)
+}