fix(vm-creation): ensure whiptail VMID inputbox is pre-filled with a valid and available ID
- Replaced hardcoded NEXTID usage with a new get_valid_nextid() function
- Ensures no collision with existing VMs, LXCs or orphaned LVM volumes
- Improves user experience by properly pre-filling whiptail inputbox
- Automatically skips invalid IDs instead of failing on alloc
c9aad3a54d