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/traefik/rotate-logs.nix

@@ -1,14 +1,14 @@
-_:
+{ vars, ... }:
 
 {
   services.logrotate = {
     enable = true;
 
     settings = {
-      "/mnt/docker/volumes/traefik-data/logs/*.log" = {
+      "${vars.nfsShares.dockerVolumes.mountpoint}/traefik-data/logs/*.log" = {
         daily = true;
-        size = "100M";
-        rotate = 20;
+        size = vars.traefikLogRotate.maxSize;
+        rotate = vars.traefikLogRotate.keep;
         compress = true;
         missingok = true;
         notifempty = true;