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.

home/rofi.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  programs.rofi = {
+    enable = true;
+    theme = "arthur";
+    font = "sans-serif";
+    package = pkgs.rofi;
+    modes = [
+      "drun"
+      "run"
+      "window"
+      "ssh"
+    ];
+    extraConfig = {
+      show-icons = true;
+    };
+  };
+}