fix: wire vars.nfsShares.options and correct stale share key names

- Filter non-attrset values from nfsShares in server.nix so the
  poolDatasets loop skips the new `options` string entry
- Fix typo proxomoxLxcImages → proxmoxLxcImages in server.nix exports
- Rename proxmoxPxeImages → pxebootImages in mount-pxe-images.nix and
  pxe-boot.nix to match the actual key in variables.nix

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/build-types/pxe-boot.nix

@@ -171,7 +171,7 @@ in
     tmpfiles.rules = [
       "d ${pxeRoot} 0755 root root -"
       "d ${httpRoot} 0755 root root -"
-      "L+ ${httpRoot}/images - - - - ${vars.nfsShares.proxmoxPxeImages.mountpoint}"
+      "L+ ${httpRoot}/images - - - - ${vars.nfsShares.pxebootImages.mountpoint}"
       "d ${httpRoot}/auto-installer 0755 root root -"
       "d ${httpRoot}/nixos-minimal 0755 root root -"
       "d ${httpRoot}/systemrescue 0755 root root -"