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/common/aliases.nix

@@ -1,4 +1,4 @@
-{ config, pkgs, lib, ... }:
+{ config, pkgs, lib, vars, ... }:
 
 let
   # Flake attribute names are now <platform>-<buildtype> (e.g. proxmox-docker)
@@ -9,13 +9,13 @@ let
     sudo nixos-rebuild switch \
       --no-write-lock-file \
       --refresh \
-      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(cat /etc/flake-target)
+      --flake git+https://${vars.lanDomain}/beatzaplenty/nixos.git#$(cat /etc/flake-target)
   '';
   myTestCmd = ''
     sudo nixos-rebuild test \
       --no-write-lock-file \
       --refresh \
-      --flake git+https://gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git#$(cat /etc/flake-target)
+      --flake git+https://${vars.lanDomain}/beatzaplenty/nixos.git#$(cat /etc/flake-target)
   '';
 in
 {