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.

hosts/hivebrain/home.nix

@@ -0,0 +1,25 @@
+{ ... }:
+
+{
+  imports = [
+    ../../home/zsh.nix
+    ../../home/fonts.nix
+    ../../home/rofi.nix
+    ../../home/yazi.nix
+    ../../home/symlinks.nix
+    ../../home/packages-base.nix
+    ../../home/packages-marvin.nix
+  ];
+
+  home.username = "marvin";
+  home.homeDirectory = "/home/marvin";
+  home.stateVersion = "26.05";
+
+  programs.git = {
+    enable = true;
+    settings.user = {
+      name = "MDietermann";
+      email = "marvin.dietermann@hive-brain.de";
+    };
+  };
+}