Update to (somewhat?) dendritic pattern

Heavily inspired by https://github.com/Doc-Steve/dendritic-design-with-flake-parts.

> Atomic commits? Never heard of them!

modules/nix/assets.nix

@@ -0,0 +1,8 @@
+{
+  flake-file.inputs = {
+    assets = {
+      url = "path:./assets";
+      flake = false;
+    };
+  };
+}