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/i18n.js

@@ -0,0 +1,25 @@
+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+import LanguageDetector from "i18next-browser-languagedetector";
+
+import enTranslation from "./locales/en/translation.json";
+import hiTranslation from "./locales/hi/translation.json";
+
+i18n
+  .use(LanguageDetector)
+  .use(initReactI18next)
+  .init({
+    resources: {
+      en: { translation: enTranslation },
+      hi: { translation: hiTranslation },
+    },
+    fallbackLng: "en",
+    supportedLngs: ["en", "hi"],
+    interpolation: { escapeValue: false }, // React handles XSS
+    detection: {
+      order: ["localStorage", "navigator"],
+      caches: ["localStorage"],
+    },
+  });
+
+export default i18n;