Translate UI and user-facing messages to Ukrainian (#5)

- All frontend pages, labels, notices and errors; html lang=uk, uk-UA money format
- Brand "Assistant System" in the top bar and page title
- Backend error details returned to the UI (auth, orders, receipts,
  cash registers, Checkbox/CRM/NP errors) and CLI output
- Tests updated for the new messages

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/auth/ProtectedRoute.tsx

@@ -10,7 +10,7 @@ export function ProtectedRoute() {
   if (status === 'loading') {
     return (
       <div className="page-center">
-        <span className="spinner" aria-label="Загрузка" />
+        <span className="spinner" aria-label="Завантаження" />
       </div>
     )
   }