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/i18n/en.json

@@ -16,11 +16,14 @@
     "phoneUnavailable": "Phone unavailable",
     "viewDetails": "View details & book"
   },
-  "nav":{"home":"Home","book":"Book","pay":"Pay","profile":"Profile","bookings":"My bookings","login":"Sign in","logout":"Sign out"},"book":{"title":"Book a Service","placeholder":"Booking flow coming soon.","cta":"Book now","selectSalon":"Select a salon from the home page to book.","service":"Service","staff":"Staff","date":"Date","time":"Time","notes":"Notes","notesPlaceholder":"Optional notes","selectService":"Select service","selectStaff":"Select staff","submit":"Confirm booking","submitting":"Booking...","errors":{"fillAll":"Please fill all required fields.","invalidTime":"Invalid date or time.","generic":"Booking failed."}},"salon":{"services":"Services","staff":"Staff"},"profile":{"title":"Profile","placeholder":"Profile and bookings coming soon.","myBookings":"My bookings","noContact":"No contact info"},"bookings":{"title":"My bookings","subtitle":"Your upcoming and past appointments.","empty":"No bookings yet.","pay":"Pay now"},"paymentReturn":{"title":"Payment status","success":"Payment successful","successMessage":"Your payment was completed. You will receive a confirmation by SMS or WhatsApp.","checkStatus":"Check your email or the salon for payment status.","reference":"Reference: {{id}}","viewBookings":"View my bookings"},"auth":{"title":"Sign in","subtitle":"Enter your phone number to receive a verification code.","phone":"Phone number","channel":"Send via","sms":"SMS","whatsapp":"WhatsApp","sendCode":"Send code","sending":"Sending...","verifyTitle":"Enter code","verifySubtitle":"We sent a code to {{phone}}","code":"Verification code","verify":"Verify","verifying":"Verifying...","back":"Change number","errors":{"generic":"Something went wrong.","retryAfter":"Please wait {{seconds}} seconds before trying again."}},"locale": {
+  "nav":{"home":"Home","book":"Book","pay":"Pay","profile":"Profile","bookings":"My bookings","login":"Sign in","logout":"Sign out"},"book":{"title":"Book a Service","placeholder":"Booking flow coming soon.","cta":"Book now","selectSalon":"Select a salon from the home page to book.","service":"Service","staff":"Staff","date":"Date","time":"Time","notes":"Notes","notesPlaceholder":"Optional notes","selectService":"Select service","selectStaff":"Select staff","submit":"Confirm booking","submitting":"Booking...","errors":{"fillAll":"Please fill all required fields.","invalidTime":"Invalid date or time.","generic":"Booking failed."}},"salon":{"services":"Services","staff":"Staff","unknownStaff":"Staff {{id}}"},"profile":{"title":"Profile","placeholder":"Profile and bookings coming soon.","myBookings":"My bookings","noContact":"No contact info"},"bookings":{"title":"My bookings","subtitle":"Your upcoming and past appointments.","empty":"No bookings yet.","pay":"Pay now"},"paymentReturn":{"title":"Payment status","success":"Payment successful","successMessage":"Your payment was completed. You will receive a confirmation by SMS or WhatsApp.","checkStatus":"Check your email or the salon for payment status.","reference":"Reference: {{id}}","viewBookings":"View my bookings"},"auth":{"title":"Sign in","subtitle":"Enter your phone number to receive a verification code.","phone":"Phone number","channel":"Send via","sms":"SMS","whatsapp":"WhatsApp","sendCode":"Send code","sending":"Sending...","verifyTitle":"Enter code","verifySubtitle":"We sent a code to {{phone}}","code":"Verification code","verify":"Verify","verifying":"Verifying...","back":"Change number","errors":{"generic":"Something went wrong.","retryAfter":"Please wait {{seconds}} seconds before trying again."}},"locale": {
     "label": "Language",
     "arabic": "العربية",
     "english": "English"
   },
+  "common": {
+    "loading": "Loading..."
+  },
   "payment": {
     "title": "Payment (Beta)",
     "subtitle": "Send a Moyasar payment for an existing booking.",