feat: add react-i18next with English and Hindi locale support.

- Add i18next + react-i18next + i18next-browser-languagedetector.
- EN/HI translation files covering all UI strings across every page and component.
- Language switcher button in Navbar; choice persisted to localStorage.
- document.documentElement.lang synced to active language in App.
- Skip-nav link and #main-content anchor for keyboard accessibility.
- aria-describedby on modal dialog; page title and meta description in index.html.
- Secure page title set to 'BIS SP-21 Standards.'

web/client/src/main.jsx

@@ -1,6 +1,7 @@
 import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
+import "./i18n.js";
 import "./index.css";
 import App from "./App.jsx";