Expand variables.nix: NFS shares, ports, image size, GC/rotation, Pi host

Adds nested vars.nfsShares (subpath + mountpoint per dataset, previously
duplicated independently across server.nix's NFS exports, mount-data.nix's
client mounts, docker.nix's tmpfiles rules, traefik's log rotation path,
and hosts/server/host.nix's beszel config), vars.ports (every literal port
in modules/ and hosts/, kept as separate entries per service even where
numbers coincide so changing one can't silently change another), plus
vars.proxmoxImageSize, vars.nixCacheGcMaxAge, vars.traefikLogRotate, and
raspberryPiHost/raspiNfsPath/raspiMountpoint for the Pi's own NFS export.

Also fixes docker.nix/minimal.nix/gui.nix hardcoding the literal "nixos"
username instead of the existing vars.primaryUser, found during the sweep.

system.stateVersion is deliberately left untouched everywhere -- per
NixOS's own docs that value must stay fixed from first install, not
follow any shared variable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

modules/beszel/enable-agent.nix

@@ -4,6 +4,6 @@
   services.beszel.agent.enable = true;
   services.beszel.agent.environment = {
     #DOCKER_HOST = "tcp://docker-socket-proxy:2375";
-    HUB_URL = "http://${vars.dockerHost}.${vars.homeDomain}:${toString vars.beszelHubPort}";
+    HUB_URL = "http://${vars.dockerHost}.${vars.homeDomain}:${toString vars.ports.beszelHub}";
   };
 }