Widget: File uploads, drag-and-drop, and user data

File uploads, drag-and-drop, user data attributes, and multi-instance support for the widget.

• Drag files into the conversation: images, PDFs, documents
• Video attachments render inline with a player
• Paste images from clipboard, files upload in the background
• Pass user attributes on init: name, email, plan, account ID, custom properties
• Automatic OS detection for the customer's device
• Multiple widget instances run independently with separate config
• hideOnLaunch option to show the widget only on specific pages
• Auto-scroll to latest message, stays put when scrolled up