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/main.tsx

@@ -6,7 +6,7 @@ import '@/index.css'
 
 const rootElement = document.getElementById('root')
 if (!rootElement) {
-  throw new Error('Элемент #root не найден в index.html')
+  throw new Error('Елемент #root не знайдено в index.html')
 }
 
 createRoot(rootElement).render(