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.

modules/desktop-common.nix

@@ -5,6 +5,7 @@
   lib,
   username,
   inputs,
+  site_config,
   niri-package,
   ...
 }:
@@ -64,11 +65,11 @@
   swapDevices = [ ];
 
   # Desktop-specific Nix cache — muffin serves it, desktops consume.
-  # Base nix settings (optimise, gc, experimental-features) come from common-nix.nix.
+  # Base nix settings (optimise, gc, experimental-features) come from common.nix.
   nix.settings = {
-    substituters = [ "https://nix-cache.sigkill.computer" ];
+    substituters = [ site_config.binary_cache.url ];
     trusted-public-keys = [
-      "nix-cache.sigkill.computer-1:ONtQC9gUjL+2yNgMWB68NudPySXhyzJ7I3ra56/NPgk="
+      site_config.binary_cache.public_key
     ];
     netrc-file = "${../secrets/desktop/nix-cache-netrc}";
   };
@@ -337,12 +338,6 @@
       "msr"
       "btusb"
     ];
-
-    kernelParams = [
-      # 1gb huge pages
-      "hugepagesz=1G"
-      "hugepages=3"
-    ];
   };
 
   services = {
@@ -381,9 +376,6 @@
     };
   };
 
-  # EST
-  time.timeZone = "America/New_York";
-
   # Select internationalisation properties.
   i18n.defaultLocale = "en_US.UTF-8";