Update flake.lock and Helium version

Refactor nix settings to use a flat `nix.settings` structure and rename
`nixPath` to `nix-path`.

modules/core/system.nix

@@ -3,24 +3,22 @@
   inputs,
   ...
 }: {
-  nix = {
-    nixPath = ["nixpkgs=${inputs.nixpkgs}"];
-    settings = {
-      download-buffer-size = 200000000;
-      auto-optimise-store = true;
-      experimental-features = [
-        "nix-command"
-        "flakes"
-      ];
-      substituters = [
-        "https://hyprland.cachix.org"
-        "https://noctalia.cachix.org"
-      ];
-      trusted-public-keys = [
-        "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
-        "noctalia.cachix.org-1:pCOR47nnMEo5thcxNDtzWpOxNFQsBRglJzxWPp3dkU4="
-      ];
-    };
+  nix.settings = {
+    nix-path = ["nixpkgs=${inputs.nixpkgs}"];
+    download-buffer-size = 200000000;
+    auto-optimise-store = true;
+    experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+    substituters = [
+      "https://hyprland.cachix.org"
+      "https://noctalia.cachix.org"
+    ];
+    trusted-public-keys = [
+      "hyprland.cachix.org-1:a7pgxzMz7+chwVL3/pzj6jIBMioiJM7ypFP8PwtkuGc="
+      "noctalia.cachix.org-1:pCOR47nnMEo5thcxNDtzWpOxNFQsBRglJzxWPp3dkU4="
+    ];
   };
   time.timeZone = "Europe/Warsaw";
   i18n = {