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/main.tsx

@@ -2,10 +2,13 @@ import { createRoot } from "react-dom/client"
 
 import App from "./App"
 import { ErrorBoundary } from "./ErrorBoundary"
+import { LocaleProvider } from "./i18n"
 import "./index.css"
 
 createRoot(document.getElementById("root")!).render(
   <ErrorBoundary>
-    <App />
+    <LocaleProvider>
+      <App />
+    </LocaleProvider>
   </ErrorBoundary>,
 )