Major config overhaul: use custom modules, setup for multi-host config, and less boilerplate

modules/user/nix/default.nix

@@ -0,0 +1,31 @@
+{ config, lib, pkgs, inputs, ... }:
+
+let
+  caches = import inputs.secrets.caches;
+in {
+  config = {
+    nix = {
+      package = lib.mkForce pkgs.nix;
+      settings = {
+        substituters =
+          (lib.optionals (caches ? urls) caches.urls) ++
+          [
+            "https://cache.nixos.org"
+            "https://hyprland.cachix.org"
+            "https://nix-community.cachix.org"
+          ];
+        trusted-public-keys =
+          (lib.optionals (caches ? publicKeys) caches.publicKeys) ++
+          [
+            "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+            "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
+            "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+          ];
+        trusted-users = [ "@wheel" ];
+        auto-optimise-store = true;
+        download-buffer-size = 500000000;
+      };
+    };
+    home.stateVersion = "22.11";
+  };
+}