refactor: moved nixos modules to dendrite pattern

modules/nixos/programs/sync/storage.nix

@@ -0,0 +1,34 @@
+{...}: {
+  flake.nixosModules.sync-storage = {
+    config,
+    lib,
+    ...
+  }: let
+    mountDir = "/mnt/sync";
+    configDir = "/etc/syncthing";
+  in {
+    options = {
+      services.syncthing.impermanence.enable = lib.mkOption {
+        type = lib.types.bool;
+        default = config.services.syncthing.enable && config.storage.impermanence.enable;
+      };
+    };
+
+    config = lib.mkIf config.services.syncthing.enable {
+      storage.datasets.replicate."system/root" = {
+        directories = {
+          "${mountDir}" = lib.mkIf config.services.syncthing.impermanence.enable {
+            enable = true;
+            owner.name = "syncthing";
+            group.name = "syncthing";
+          };
+          "${configDir}" = lib.mkIf config.services.syncthing.impermanence.enable {
+            enable = true;
+            owner.name = "syncthing";
+            group.name = "syncthing";
+          };
+        };
+      };
+    };
+  };
+}