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/api/v1/auth.py

@@ -70,7 +70,7 @@ async def change_password(
 ) -> Response:
     if not verify_password(payload.current_password, user.password_hash):
         raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST, detail="Текущий пароль указан неверно"
+            status_code=status.HTTP_400_BAD_REQUEST, detail="Поточний пароль вказано неправильно"
         )
 
     user.password_hash = hash_password(payload.new_password)