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

nix/tests/default.nix

@@ -0,0 +1,20 @@
+{ self, pkgs }: {
+  testServiceStarts = pkgs.testers.runNixOSTest (_: {
+    name = "service-starts";
+    nodes = {
+      machine = _: {
+        imports = [
+          self.nixosModules.default
+        ];
+
+        services.yt-dlp-web-ui = {
+          enable = true;
+          downloadDir = "/var/lib/yt-dlp-web-ui";
+        };
+      };
+    };
+    testScript = ''
+      machine.wait_for_unit("yt-dlp-web-ui")
+    '';
+  });
+}