I am looking for a developer to build a professional-grade user interface for a Google Sheets project. You will be responsible for the "Front-End" of a data-entry tool. ​Key Tasks: ​Sidebar Development: Create a clean, modern Sidebar using Html Service (HTML/CSS/JS). ​Data Extraction: Write logic to scrape data from specific columns in a Google Sheet (e.g., Date, Amount, Account Code, Description). ​The "Handshake": Your script will NOT process the data. Instead, it must package the data into a clean JSON object and send it via UrlFetchApp to a specific Web App URL (to be provided). ​Dynamic Templates: The sidebar must include a "Settings" section where users can enter credentials. It must also feature a "Validate" button that checks for empty fields before sending. ​Requirements: ​Expert in Google Apps Script and HtmlService. ​Strong CSS skills to make the sidebar feel like a modern SaaS product. ​Ability to handle asynchronous callbacks (showing a "Loading" spinner in the sidebar while waiting for the URL response).