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.

services/jellyfin/jellyfin.nix

@@ -1,6 +1,7 @@
 {
   pkgs,
   config,
+  site_config,
   service_configs,
   lib,
   ...
@@ -24,7 +25,7 @@
     inherit (service_configs.jellyfin) dataDir cacheDir;
   };
 
-  services.caddy.virtualHosts."jellyfin.${service_configs.https.domain}".extraConfig = ''
+  services.caddy.virtualHosts."jellyfin.${site_config.domain}".extraConfig = ''
     reverse_proxy :${builtins.toString service_configs.ports.private.jellyfin.port} {
       # Disable response buffering for streaming. Caddy's default partial
       # buffering delays fMP4-HLS segments and direct-play responses where