refactor(nix): migrate from flake-utils to flake-parts

Replace flake-utils with flake-parts for a more modular and type-safe
Nix flake configuration. flake-parts uses the NixOS module system
pattern, providing better extensibility and composability.

Key changes:
- Replace flake-utils input with flake-parts
- Use mkFlake with perSystem pattern instead of eachDefaultSystem
- Explicitly declare supported systems (x86_64-linux, aarch64-linux,
  aarch64-darwin)
- Remove x86_64-darwin from supported systems as it is no longer needed

Benefits of flake-parts:
- NixOS-style module system for flake configuration
- Type-safe options with better error messages
- Support for advanced features like partitions and flakeModules
- More maintainable structure for complex flake configurations

Author: ryoppippi

flake.lock

@@ -3,15 +3,18 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
-    flake-utils.url = "github:numtide/flake-utils";
+    flake-parts.url = "github:hercules-ci/flake-parts";
   };
 
-  outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
-      let
-        pkgs = nixpkgs.legacyPackages.${system};
-      in
-      {
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "aarch64-darwin"
+      ];
+
+      perSystem = { pkgs, ... }: {
         devShells.default = pkgs.mkShell {
           buildInputs = with pkgs; [
             pnpm_10
@@ -27,6 +30,6 @@
             fi
           '';
         };
-      }
-    );
+      };
+    };
 }