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>

variables.nix

@@ -0,0 +1,21 @@
+{
+  # Network / domains
+  lanDomain = "gitea.lan.ddnsgeek.com"; # Gitea/DDNS domain
+  homeDomain = "sweet.home"; # base LAN domain for service subdomains (pve., docker.)
+  tailnetDomain = "tail13f623.ts.net"; # Tailscale MagicDNS suffix
+  lanCidr = "192.168.2.0/24"; # LAN subnet
+  pxeServerIp = "192.168.2.247"; # pxe-boot host's LAN IP
+  pbsIp = "192.168.2.108"; # Proxmox Backup Server LAN IP
+
+  # Cross-host references (LAN hostnames/users other hosts reach over the network)
+  nixCacheHost = "nix-cache"; # substituter/remote-builder hostname
+  nfsServerHost = "server"; # NFS export source hostname
+  remoteBuilderUser = "nixremote"; # remote builder SSH user
+
+  # System
+  timeZone = "Australia/Brisbane";
+  primaryUser = "nixos"; # main interactive user on every host
+
+  # Storage
+  storageRoot = "/tank"; # ZFS pool root on `server`
+}