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>

hosts/nix-cache/host.nix

@@ -1,7 +1,7 @@
-{ config, ... }:
+{ config, vars, ... }:
 
 {
-  networking.hostName = "nix-cache";
+  networking.hostName = vars.nixCacheHost;
 
   sops.secrets."beszel-token".sopsFile = ../../secrets/nix-cache.yaml;
   sops.templates."nix-cache-beszel.env".content = ''