feat: add flake.nix for Nix-based installation

flake.nix

@@ -0,0 +1,61 @@
+{
+  description = "eitango - English vocabulary learning CLI";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        packages = {
+          eitango = pkgs.buildGoModule {
+            pname = "eitango";
+            version = self.shortRev or self.dirtyShortRev or "dev";
+
+            src = self;
+
+            vendorHash = "sha256-fVqXrhz2wMWvWiPL8X/M0Oqz6LIRSLocKNZmEVhQLtY=";
+
+            subPackages = [ "cmd/eitango" ];
+
+            env.CGO_ENABLED = 0;
+
+            ldflags = [
+              "-s"
+              "-w"
+              "-X main.version=${self.shortRev or self.dirtyShortRev or "dev"}"
+              "-X main.commit=${self.rev or "dirty"}"
+              "-X main.date=${self.lastModifiedDate or "1970-01-01T00:00:00Z"}"
+            ];
+
+            meta = {
+              description = "English vocabulary learning CLI with spaced repetition";
+              homepage = "https://github.com/harumiWeb/eitango";
+              license = pkgs.lib.licenses.asl20;
+              mainProgram = "eitango";
+            };
+          };
+          default = self.packages.${system}.eitango;
+        };
+
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            go
+            gopls
+            golangci-lint
+          ];
+        };
+      }
+    );
+}