Add Nix (#177)

* add Nix support

* fix formatter output

* mention Nix in README

* fix common import

* fix frontend old version import

* clarified flake pkgs order

* rm old dataDir option

* comment typo

* fix password assertion

* rm old User/Group logic

* rewrite assertion boolean expr

* General flake touchup

- Rewrite `callPackage` exprs to be more readable
- Add pre-commit support for devShell
- Add direnv support

* add simple test

* use correct test func

flake.nix

@@ -0,0 +1,51 @@
+{
+  description = "A terrible web ui for yt-dlp. Designed to be self-hosted.";
+
+  inputs = {
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    pre-commit-hooks-nix.url = "github:cachix/pre-commit-hooks.nix";
+  };
+
+  outputs = inputs@{ self, flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [
+        inputs.pre-commit-hooks-nix.flakeModule
+      ];
+      systems = [
+        "x86_64-linux"
+      ];
+      perSystem = { config, self', pkgs, ... }: {
+
+        packages = {
+          yt-dlp-web-ui-frontend = pkgs.callPackage ./nix/frontend.nix { };
+          default = pkgs.callPackage ./nix/server.nix {
+            inherit (self'.packages) yt-dlp-web-ui-frontend;
+          };
+        };
+
+        checks = import ./nix/tests { inherit self pkgs; };
+
+        pre-commit = {
+          check.enable = true;
+          settings = {
+            hooks = {
+              ${self'.formatter.pname}.enable = true;
+              deadnix.enable = true;
+              nil.enable = true;
+              statix.enable = true;
+            };
+          };
+        };
+
+        devShells.default = pkgs.callPackage ./nix/devShell.nix {
+          inputsFrom = [ config.pre-commit.devShell ];
+        };
+
+        formatter = pkgs.nixpkgs-fmt;
+      };
+      flake = {
+        nixosModules.default = import ./nix/module.nix self.packages;
+      };
+    };
+}