[feature request] Add URL parameter to load custom SB3 file

[HackerQED]

Hello Scratch GUI team,

I'd like to propose a new feature that would allow loading a custom SB3 file via URL parameter when the Scratch GUI loads. This would be extremely useful for developers who want to create custom Scratch-based applications or educational platforms.

Currently, it seems there's no built-in way to load a specific SB3 file on startup via URL. Adding this feature would enable scenarios like:

1. Creating custom learning paths with pre-loaded projects
2. Easily sharing specific projects for collaboration
3. Integrating Scratch projects into other web applications

The use case example could be:

http://my-site-using-scratch-gui.com/?sb3Url=https://assets.my-site-using-scratch-gui.com/my-project.sb3

This feature would greatly enhance the flexibility of Scratch GUI for developers building on top of it. If there's interest from the community, I'd be happy to work on a pull request to implement this functionality.

What do you think about this feature? Is it something that could be valuable for the project?

Thank you for your attention!