fix(nix-cache): use FQDN to fix hostname resolution on clients

systemd-resolved only uses LLMNR for single-label hostnames, never DNS —
same issue mount-data.nix already documented and fixed for NFS by switching
to server.sweet.home. Change the substituter URL, SSH knownHosts, and
remote-builder hostName from bare "nix-cache" to "nix-cache.sweet.home",
and update nginx's virtualHost to match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/nix-cache/client.nix

@@ -3,7 +3,7 @@
 {
   nix.settings = {
     substituters = [
-      "http://${vars.nixCacheHost}"
+      "http://${vars.nixCacheHost}.${vars.homeDomain}"
       "https://cache.nixos.org/"
     ];
     trusted-public-keys = [