Split configuration.nix and home.nix into per-domain modules

Break the two monolithic files into focused modules under system/
and home/ so each concern (boot, networking, xserver, picom, zsh,
git, rofi, etc.) lives in its own file. Clean up duplicates: drop
rofi from home.packages since programs.rofi.enable already installs
it, and move user-only apps (alacritty, obsidian, claude-code) from
environment.systemPackages to home.packages. Remove commented-out
boilerplate carried over from the install template.

system/users.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+{
+  users.users.marvin = {
+    isNormalUser = true;
+    extraGroups = [ "wheel" ];
+    shell = pkgs.zsh;
+    packages = with pkgs; [
+      tree
+    ];
+  };
+}