Add Nix flake for building KeSp_controller

flake.nix

@@ -0,0 +1,56 @@
+{
+  description = "KeSp split keyboard configurator — Slint UI";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+
+        nativeBuildInputs = with pkgs; [
+          pkg-config
+          cmake
+        ];
+
+        buildInputs = with pkgs; [
+          fontconfig
+          libxkbcommon
+          wayland
+          udev
+        ] ++ pkgs.lib.optionals pkgs.stdenv.isLinux [
+          xorg.libX11
+          xorg.libXcursor
+          xorg.libXrandr
+          xorg.libXi
+        ];
+      in
+      {
+        packages.default = pkgs.rustPlatform.buildRustPackage {
+          pname = "kesp-controller";
+          version = "1.0.0";
+          src = ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+
+          inherit nativeBuildInputs buildInputs;
+
+          # Slint needs to find fontconfig at runtime
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath buildInputs;
+
+          meta = with pkgs.lib; {
+            description = "Cross-platform configurator for the KeSp split ergonomic keyboard";
+            license = licenses.gpl3Only;
+            mainProgram = "KeSp_controller";
+          };
+        };
+
+        devShells.default = pkgs.mkShell {
+          inherit nativeBuildInputs buildInputs;
+          LD_LIBRARY_PATH = pkgs.lib.makeLibraryPath buildInputs;
+          packages = with pkgs; [ cargo rustc rust-analyzer clippy ];
+        };
+      });
+}