Centralize shared values into variables.nix

One file (variables.nix) holding every value that was previously
hardcoded and repeated across modules: LAN domain/CIDR, home/tailnet
domains, cross-host references (nix-cache substituter hostname, NFS
server hostname, remote-builder user), PXE/PBS IPs, timezone, and the
primary username.

Wired in via flake.nix's specialArgs (and home-manager's
extraSpecialArgs for the two home.nix files), so any module picks it
up by just adding `vars` to its function arguments — no explicit
import needed. Two hosts (nix-cache, server) now derive their own
networking.hostName from the same variable other hosts use to reach
them, so there's exactly one place to change either identifier.

Purely mechanical: every substituted value matches what was already
there, confirmed by identical toplevel .drv paths for all 17 targets
before and after.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

modules/services/docker-health-to-gotify.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, vars, ... }:
 
 {
   systemd.services.docker-health-to-gotify = {
@@ -7,9 +7,9 @@
     serviceConfig = {
       Type = "oneshot";
       # Run as root so it can read /etc/secrets and access docker socket
-     # User = "root";
+      # User = "root";
       #EnvironmentFile = "-/etc/secrets/docker-health-alert.env";
-      ExecStart = "${pkgs.bash}/bin/bash /home/nixos/docker/monitoring/gotify/docker-health-to-gotify.sh";
+      ExecStart = "${pkgs.bash}/bin/bash /home/${vars.primaryUser}/docker/monitoring/gotify/docker-health-to-gotify.sh";
       StandardOutput = "journal";
       StandardError = "journal";
     };
@@ -25,4 +25,4 @@
       Persistent = true;
     };
   };
-}
+}