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>

backend/app/services/checkbox/client.py

@@ -63,7 +63,7 @@ def get_checkbox_client() -> CheckboxClient:
     """Один экземпляр на процесс: внутри — кэш токенов кассиров."""
     if settings.checkbox_use_stub:
         if settings.is_production:
-            raise RuntimeError("CHECKBOX_USE_STUB=true запрещён в production")
+            raise RuntimeError("CHECKBOX_USE_STUB=true заборонено в production")
         from app.services.checkbox.stub_client import StubCheckboxClient
 
         return StubCheckboxClient(auto_complete_after=2)