feat: add Arabic translations and fix frontend i18n gaps

- Add backend/locale/ar_SA/LC_MESSAGES/django.po with Arabic (ar-sa) translations
  for all 62 user-facing error/validation strings across accounts, bookings,
  payments, and notifications apps; compile to django.mo
- Add common.loading and salon.unknownStaff keys to both ar-sa.json and en.json
- ProtectedRoute: replace hardcoded "Loading..." with t("common.loading")
- BookPage, SalonDetailPage: replace `Staff ${s.id}` fallback with
  t("salon.unknownStaff", { id: s.id })
- BookingsPage: pass getActiveLocale() to toLocaleString so date/time
  format matches the active app language

All 35 backend tests and 7 frontend tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ProtectedRoute.jsx

@@ -1,14 +1,16 @@
 import { Navigate, useLocation } from "react-router-dom";
+import { useTranslation } from "react-i18next";
 import { useAuth } from "../contexts/AuthContext";
 
 export default function ProtectedRoute({ children }) {
+  const { t } = useTranslation();
   const { isAuthenticated, loading } = useAuth();
   const location = useLocation();
 
   if (loading) {
     return (
       <div className="auth-loading">
-        <p>Loading...</p>
+        <p>{t("common.loading")}</p>
       </div>
     );
   }