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

Drop files into conversations, pass user data for context, and control when the widget shows up.

What's new

• Drag-and-drop file uploads and video attachment rendering in chat messages.
• Pass user attributes to the widget for richer context, with automatic OS detection.
• Instance management for multiple widgets, plus a hideOnLaunch option to start hidden.
• Smoother scrolling and improved file handling in the rich text editor.