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/AuthContext.tsx

@@ -65,7 +65,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
       setUser(result.user)
       setStatus('authenticated')
     } catch (err) {
-      const message = err instanceof ApiError ? err.message : 'Не удалось подключиться к серверу'
+      const message = err instanceof ApiError ? err.message : 'Не вдалося підключитися до сервера'
       setError(message)
       throw err
     }