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.

home/progs/zen/dark-reader.nix

@@ -68,19 +68,19 @@ in
       "element.envs.net"
       "mail.proton.me"
       "mail.google.com"
-      "www.gardling.com"
+      "www.sigkill.computer"
       "projects.fivethirtyeight.com"
       "secure.bankofamerica.com"
       "billpay-ui.bankofamerica.com"
       "plus.pearson.com"
-      "immich.gardling.com"
+      "immich.sigkill.computer"
       "huggingface.co"
       "session.masteringphysics.com"
       "brainly.com"
       "www.270towin.com"
       "phet.colorado.edu"
       "8042-1.portal.athenahealth.com"
-      "torrent.gardling.com"
+      "torrent.sigkill.computer"
       "nssb-p.adm.fit.edu"
       "mail.openbenchmarking.org"
       "moneroocean.stream"
@@ -89,11 +89,11 @@ in
       "chat.deepseek.com"
       "n21.ultipro.com"
       "www.egaroucid.nyanyan.dev"
-      "bitmagnet.gardling.com"
+      "bitmagnet.sigkill.computer"
       "frame.work"
       "www.altcancer.net"
       "jenkins.jpenilla.xyz"
-      "soulseek.gardling.com"
+      "soulseek.sigkill.computer"
       "discord.com"
       "www.lufthansa.com"
       "surveys.hyundaicx.com"