fix(ipam): require resource name before allocating IP and fill missing fields on VM link

- Validate resource name in auto-allocate form before sending request
- Fill resource_name, proxmox_node, resource_type, mac_address when syncing
  a VM that already has matching vmid/server_id but missing metadata

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

frontend/src/features/ipam/IPAMToolsPage.tsx

@@ -97,6 +97,7 @@ function AllocateCard() {
   };
   const handleAllocate = () => {
     if (!networkId) { toast.error(t('ipam.select_network')); return; }
+    if (!resourceName.trim()) { toast.error(t('ipam.resource_required')); return; }
     autoAllocate.mutate(
       { network_id: Number(networkId), resource_name: resourceName || undefined, hostname: hostname || undefined },
       {