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

modules/installer/iso.nix

@@ -21,11 +21,20 @@
     settings.PermitRootLogin = "prohibit-password";
   };
 
-  users.motd = ''
+  users.motd =
+    if config.router.installer.system != null then
+      ''
 
-    nixos-router installer
-      1. (optional) scp -r secrets root@<this-host>:secrets
-      2. router-install --secrets /root/secrets /dev/<disk>
-    The disk is erased after confirmation.
-  '';
+        nixos-router installer
+          1. (optional) scp -r secrets root@<this-host>:secrets
+          2. router-install --secrets /root/secrets /dev/<disk>
+        The disk is erased after confirmation.
+      ''
+    else
+      ''
+
+        nixos-router installer
+          Run `sudo router-setup`: it asks a few questions, erases the chosen disk
+          and installs the router. Everything else is set up in the web UI.
+      '';
 }