File attachment restyle and drag-and-drop

Working with file attachments in the AI agent chat is smoother. Drag any file — image, PDF, spreadsheet, document — anywhere on the page to attach it, or use the existing attach button as before. Attachments display as consistent cards in both your message and in chat history, with thumbnails for images and clear filenames for documents. Click an attached image to open it full-screen for a closer look.