feat!: change machine hostnames

Instead of merely naming my systems "laptop", "desktop", etc. (boring),
I now have them named after Aristotelian elements (cool).

This also gives me an excuse to refactor my flake file.

config/battery.nix

@@ -1,5 +1,5 @@
 { config, lib, ... }:
-lib.mkIf (config.platform == "laptop") {
+lib.mkIf (config.machine == "fire") {
   services.upower = {
     enable = true;