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/disko/proxmox.nix

@@ -1,4 +1,4 @@
-{ config, ... }:
+{ config, vars, ... }:
 
 {
   disko.devices = {
@@ -13,7 +13,7 @@
       # whatever's left after ESP + swap within it); imageName keeps each
       # host's image distinctly named instead of every proxmox-* host
       # producing an identical "main.raw".
-      imageSize = "20G";
+      imageSize = vars.proxmoxImageSize;
       imageName = config.networking.hostName;
 
       content = {