refactor: moved nixos modules to dendrite pattern

modules/nixos/programs/sync/sync.nix

@@ -0,0 +1,38 @@
+{...}: {
+  flake.nixosModules.sync-service = {
+    config,
+    lib,
+    syncthingConfiguration,
+    ...
+  }: let
+    mountDir = "/mnt/sync";
+    configDir = "/etc/syncthing";
+  in {
+    config = lib.mkMerge [
+      {
+        systemd = lib.mkIf config.services.syncthing.enable {
+          tmpfiles.rules = [
+            "A ${mountDir} -    -        -               - u:syncthing:rwX,g:syncthing:rwX,o::-"
+            "d ${mountDir} 2755 syncthing syncthing -"
+            "d ${config.services.syncthing.dataDir} 775 syncthing syncthing -"
+            "d ${config.services.syncthing.configDir} 755 syncthing syncthing -"
+          ];
+        };
+      }
+      (lib.mkIf config.services.syncthing.enable (lib.mkMerge [
+        {
+          services.syncthing = {
+            user = "syncthing";
+            group = "syncthing";
+            dataDir = "${mountDir}/default";
+            configDir = configDir;
+            overrideDevices = true;
+            overrideFolders = true;
+            configuration = syncthingConfiguration;
+            deviceName = config.networking.hostName;
+          };
+        }
+      ]))
+    ];
+  };
+}