site-config: dedupe cross-host values, fix stale dark-reader urls, drop desktop 1g hugepages

new site-config.nix holds values previously duplicated across hosts:
  domain, old_domain, contact_email, timezone, binary_cache (url + pubkey),
  dns_servers, lan (cidr + gateway), hosts.{muffin,yarn} (ip/alias/ssh_host_key),
  ssh_keys.{laptop,desktop,ci_deploy}.

threaded through specialArgs on all three hosts + home-manager extraSpecialArgs +
homeConfigurations.primary + serverLib. service-configs.nix now takes
{ site_config } as a function arg and drops its https namespace; per-service
domains (gitea/matrix/ntfy/mollysocket/livekit/firefox-sync/grafana) are
derived from site_config.domain. ~15 service files and 6 vm tests migrated.

breakage fixes rolled in:
 - home/progs/zen/dark-reader.nix: 5 stale *.gardling.com entries in
   disabledFor rewritten to *.sigkill.computer (caddy 301s the old names so
   these never fired and the new sigkill urls were getting dark-reader applied)
 - modules/desktop-common.nix: drop unused hugepagesz=1G/hugepages=3
   kernelParams (no consumer on mreow or yarn; xmrig on muffin still reserves
   its own via services/monero/xmrig.nix)

verification: muffin toplevel is bit-identical to pre-refactor baseline.
mreow/yarn toplevels differ only in boot.json kernelParams + darkreader
storage.js (nix-diff verified). deployGuardTest and fail2banVaultwardenTest
(latter exercises site_config.domain via bitwarden.nix) pass.

lib/default.nix

@@ -2,6 +2,7 @@
   inputs,
   pkgs,
   service_configs,
+  site_config,
   lib ? inputs.nixpkgs-stable.lib,
   ...
 }:
@@ -195,7 +196,7 @@ lib.extend (
       assert (subdomain != null) != (domain != null);
       { config, ... }:
       let
-        vhostDomain = if domain != null then domain else "${subdomain}.${service_configs.https.domain}";
+        vhostDomain = if domain != null then domain else "${subdomain}.${site_config.domain}";
         upstream =
           if vpn then
             "${config.vpnNamespaces.wg.namespaceAddress}:${builtins.toString port}"