feat(linux): Allow running without root

[darkwater]

Closes #1354

An additional "continue anyway" button in the dialog might be nice too, but at least this now provides the possibility of running it without root.

[tdewey-rpi]

Thanks for the submission, @darkwater.

I'm not inclined to accept this.

Where I have explained why Imager behaves in this manner, the primary point is that failing early is better than failing later. Simply adding a way to circumvent the early failure, but without engaging the later failure, is not likely to result in a net confusion reduction. If anything, I could see an enterprising repackager put this flag into their .desktop file, and then create untold confusion in their user base.

To run without root, the application must be fractured into privileged and unprivileged parts.

[darkwater]

I had Copilot add some UI to indicate unwritable devices. Would something like this suffice?

Running GUI applications as root is generally not a great thing. In fact, Wayland compositors normally don't allow connections from other users, even from root. If I do sudo rpi-imager right now, it simply doesn't work. I understand wanting to avoid confusion, but this cannot come at the cost of functionality.