feat(xdg): add persistence and configure paths

The default xdg directories are noisy and unhelpful.

home/xdg/home.nix

@@ -0,0 +1,21 @@
+{ config
+, ... }: {
+  xdg = {
+    enable = true;
+    userDirs = let
+      home = config.home.homeDirectory;
+      local = "${home}/local";
+    in {
+      enable = true;
+      createDirectories = true;
+      download = "${home}/dl";
+      desktop = "${local}/desktop";
+      documents = "${local}/documents";
+      music = "${local}/music";
+      pictures = "${local}/pictures";
+      publicShare = "${local}/public";
+      templates = "${local}/templates";
+      videos = "${local}/videos";
+    };
+  };
+}

home/xdg/nixos.nix

@@ -0,0 +1,3 @@
+{
+  users.home.persist.directories = [ "local" "dl" ];
+}