Restructure flake into per-host modules

Move host-specific files under hosts/<name>/ so a second machine can be
added without forking the repo. Split system/users.nix and
system/display-manager.nix into per-option subfolders, separate the
home-manager packages list into a shared base and a personal bundle,
extract networking.hostName out of the shared networking module, and
inline the git identity into the per-host home.nix. Add .gitignore
guardrails to keep secrets out.

system/users/marvin.nix

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