Add a generic installer that asks for the basics and builds offline

The installer ISO no longer needs a prebuilt system: `router-setup` asks
for the disk, the WAN and LAN interfaces, DHCP or PPPoE (optionally on a
VLAN), host name, LAN address and a password, writes settings.json and
the secrets, builds that router offline (the ISO carries a reference
router system whose closure has everything the build needs) and installs
it with `router-install --system`.

The password is used for the web UI and, hashed, as root's console
password (routerSettings sets root's hashedPasswordFile; it applies when
the account is created at installation).

checks.setup answers router-setup on the console and checks the installed
box: its settings, the web UI and console login with that password, and
a rebuild that reproduces the installed system.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

tests/default.nix

@@ -11,6 +11,7 @@
   dns = import ./dns.nix { inherit self pkgs; };
   installer = import ./installer.nix { inherit self pkgs; };
   apply = import ./apply.nix { inherit self pkgs; };
+  setup = import ./setup.nix { inherit self pkgs; };
   image = import ./image.nix { inherit self pkgs; };
   wireguard = import ./wireguard.nix { inherit self pkgs; };
   lib = import ./lib.nix { inherit pkgs; };