Feature Request: Drag & Drop File Support for Multipass Shell

It would be extremely useful to support drag and drop of files from the host system directly into the Multipass shell window. When a user drags a file into the shell, it should be automatically transferred into the instance — ideally to a predefined directory such as /home/ubuntu/Downloads.

This feature would streamline quick file transfers, particularly in development, testing, or demo workflows:

- Removes the need to manually configure scp or use multipass transfer for simple file operations

- Speeds up common tasks like copying scripts, configs, or data files

- Makes Multipass more accessible for users who prefer GUI-based interactions

Start by tracing the Multipass shell entry point and how host files are transferred into an instance, comparing the proposed drag-and-drop flow with scp or multipass transfer. Done means a file dragged from the host into the shell is transferred automatically to the agreed instance directory, with behavior suitable for development, testing, and demo workflows.

Written by the indexing model from the issue text.