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/index.css

@@ -1,3 +1,28 @@
+/* ── Skip navigation (accessibility) ── */
+.skip-nav {
+  position: absolute;
+  left: -9999px;
+  top: auto;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+  z-index: 9999;
+}
+.skip-nav:focus {
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: auto;
+  height: auto;
+  padding: 0.75rem 1.5rem;
+  background: var(--accent, #d4530a);
+  color: #fff;
+  font-weight: 600;
+  text-decoration: none;
+  border-radius: 0 0 4px 0;
+  outline: 3px solid #fff;
+}
+
 /* ── BIS SP-21 Design Tokens ── */
 :root {
   --accent:          #d4530a;