[UI] System Backdrop for the login screen

Feature Request

Short description of the proposed feature

For a little while, Foundry has allowed the definition of a default background for game systems to replace the grey denim background. This can be superseded by defining a background in the World config.

Proposed implementation

1. Create a background for CONAN in a sufficiently large resolution to still be pretty on 4K monitors.
2. In the system.json file, add a "background": "CONAN-Header.webp entry
3. We should use a variant of that background as a banner for this repo.

Reference

The L5R5E system has implemented it.