refactor(cardwired): Correctly identify GPU types, and improve reliability

[luytan]

Description

Add a GpuType enum, improve code readability, add more reliable discrete checks and change systemd service to start after nvidia modprobe

Checklist:

• My code follows the style guidelines of this project (cargo fmt)
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the mdBook documentation
• My changes generate no new warnings (clippy/clang)
• New and existing unit tests pass locally with my changes (either use nix flake check or wait for the ci)

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