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/tests/test_nova_poshta_client.py

@@ -110,7 +110,7 @@ class TestNpTrackingClientGetStatuses:
     async def test_rejects_too_many_documents(self) -> None:
         client = NpTrackingClient()
 
-        with pytest.raises(NovaPoshtaError, match="Слишком много"):
+        with pytest.raises(NovaPoshtaError, match="Забагато"):
             await client.get_statuses(
                 api_key=API_KEY, waybill_numbers=[str(i) for i in range(101)]
             )