Unite Desktop and Laptop configs

[hyperpastel]

Currently, the desktop uses a config that was initially based on the laptop one in this repo but has since diverged, making it unable to receive changes to this one.

Instead, the config should be rewritten into a modularized form that shares core functionality among both machines but allows both to still have their own settings (needed primarily for hardware but also for differing software needs).

In the end, the flake should provide a target for each machine.

This would also be a good opportunity to sort out what packages belong to home-manager and which to the systems environment.