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_receipts_router.py

@@ -45,7 +45,9 @@ def queue(monkeypatch: pytest.MonkeyPatch) -> FakeQueue:
     )
 
     async def fake_request(session: Any, items: Any, *, user: Any, request: Any) -> Any:
-        return receipts_service.RequestResult(created=[receipt], errors={"2": "У заказа нет ТТН"})
+        return receipts_service.RequestResult(
+            created=[receipt], errors={"2": "У замовлення немає ТТН"}
+        )
 
     async def fake_commit(self: AsyncSession) -> None:
         return None
@@ -74,5 +76,5 @@ def test_cashier_creates_and_enqueues(queue: FakeQueue) -> None:
     assert response.status_code == 202
     body = response.json()
     assert [r["order_id"] for r in body["created"]] == ["1"]
-    assert body["errors"] == {"2": "У заказа нет ТТН"}
+    assert body["errors"] == {"2": "У замовлення немає ТТН"}
     assert queue.jobs == [("create_ettn_receipt", (body["created"][0]["id"],))]