Fix LXC deployment path and clean up remaining eval warnings

LXC hosts (device busy fix):

modules/platforms/lxc.nix now imports nixpkgs' own
virtualisation/proxmox-lxc.nix, giving every lxc-* host a real
config.system.build.tarball output — a directly `pct restore`-able
Proxmox container image. This is the actual bug fix behind the
"cannot remove real root directory: device busy or in use" error:
lxc-* targets were only reachable through nixos-install, which
bind-mounts / onto /mnt for containers (no raw disk to partition)
and then correctly refuses to modify the filesystem it's currently
running on. auto-install.sh's menu now excludes lxc-* targets
entirely (they deploy via nix build + pct restore instead, see
docs/auto-installer.md) — and, on the same reasoning, also excludes
`installer`/`proxmox-lxc`, which are the installer image's own flake
targets, not deployable hosts.

manageHostName = true keeps host.nix's declared hostnames (upstream's
default would let Proxmox's ambient container config win instead);
privileged = false matches how these containers are actually created.

Eval warnings, now zero across all 19 nixosConfigurations + 4 packages:

- Multiple password options (root/nixos in the installer): nixpkgs'
  own installer profile sets initialHashedPassword = "" for
  passwordless login, conflicting with our explicit hashedPassword.
  Force-nulled the upstream option rather than adopting passwordless
  login, since this image now also boots over LAN PXE with
  PasswordAuthentication enabled.
- boot.zfs.forceImportRoot default value: set explicitly to false
  (matching the two places that already did) in
  modules/common/configuration.nix and modules/installer/common.nix,
  covering every host and the installer alike.
- Deprecated pkgs.system in modules/build-types/gui.nix: switched to
  pkgs.stdenv.hostPlatform.system.

All confirmed non-behavioral where it matters: unrelated hosts'
drvPaths are byte-identical to their pre-existing baselines throughout.

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

modules/build-types/gui.nix

@@ -2,7 +2,7 @@
 
 {
   environment.systemPackages = with pkgs; [
-    inputs.nixos-conf-editor.packages.${pkgs.system}.nixos-conf-editor
+    inputs.nixos-conf-editor.packages.${pkgs.stdenv.hostPlatform.system}.nixos-conf-editor
     nodejs
     appimage-run
     seahorse