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/pages/SalonDetailPage.jsx

@@ -43,7 +43,7 @@ export default function SalonDetailPage() {
       <h2>{t("salon.staff")}</h2>
       <ul className="staff-list">
         {salon.staff?.map((s) => (
-          <li key={s.id}>{s.name || s.title || `Staff ${s.id}`}</li>
+          <li key={s.id}>{s.name || s.title || t("salon.unknownStaff", { id: s.id })}</li>
         ))}
       </ul>