Summary

• Adds optional subnet pool selector to the create subnet modal, filtered by the selected IP version
• CIDR becomes optional when a subnet pool is selected (pool auto-allocates)
• Reorders form fields to a logical flow: Name → IP Version → Subnet Pool → CIDR → Gateway → DHCP

Test plan

• Open subnet create modal, verify "Subnet Pool" field appears between IP Version and CIDR
• Toggle IP version and confirm pool list filters accordingly
• Select a pool and submit without CIDR — should succeed
• Submit without CIDR and without pool — should show validation error
• Submit with both CIDR and pool — should succeed