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.
ZacharyZcR