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/server.nix

@@ -20,7 +20,7 @@
     nginx = {
       enable = true;
       recommendedProxySettings = true;
-      virtualHosts.${vars.nixCacheHost} = {
+      virtualHosts."${vars.nixCacheHost}.${vars.homeDomain}" = {
         locations."/" = {
           proxyPass = "http://${config.services.nix-serve.bindAddress}:${toString config.services.nix-serve.port}";
         };