fix(build): validate storage availability when loading defaults

When loading var_container_storage / var_template_storage from default.vars
or app defaults, validate that the storage actually exists and is active on
the current node. In mixed clusters (e.g. LVM-Thin + ZFS hosts), saved
defaults from one node type would fail on another because the referenced
storage doesn't exist. Now invalid storage values are skipped with a
warning, allowing the normal storage selection prompt to appear.

Closes #12766

misc/build.func

@@ -1185,6 +1185,18 @@ load_vars_file() {
             continue
           fi
           ;;
+        var_container_storage | var_template_storage)
+          # Validate that the storage exists and is active on the current node
+          local _storage_status
+          _storage_status=$(pvesm status 2>/dev/null | awk -v s="$var_val" '$1 == s { print $3 }')
+          if [[ -z "$_storage_status" ]]; then
+            msg_warn "Storage '$var_val' from $file not found on this node, ignoring"
+            continue
+          elif [[ "$_storage_status" == "disabled" ]]; then
+            msg_warn "Storage '$var_val' from $file is disabled on this node, ignoring"
+            continue
+          fi
+          ;;
         esac
       fi