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/pxe-boot/mount-pxe-images.nix

@@ -7,8 +7,8 @@ let
   nfsServer = "${vars.nfsServerHost}.${vars.homeDomain}";
 in
 {
-  fileSystems.${vars.nfsShares.proxmoxPxeImages.mountpoint} = {
-    device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.proxmoxPxeImages.subpath}";
+  fileSystems.${vars.nfsShares.pxebootImages.mountpoint} = {
+    device = "${nfsServer}:${vars.storageRoot}/${vars.nfsShares.pxebootImages.subpath}";
     fsType = "nfs";
     options = [
       "_netdev"