Let the Reserve button pick another address of the lease's subnet

The Reserve dialog fills in the lease's address but lets it be changed;
POST /leases/{ip}/reserve takes an optional ip, steward lease reserve
takes --ip. The address must be in the lease's subnet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/subnets.md

@@ -56,6 +56,7 @@ A reservation fixes a client's address:
 ```sh
 steward reservation add 10.0.10.20 aa:bb:cc:dd:ee:01 --hostname nas
 steward lease reserve 10.0.10.57       # the address a client has now
+steward lease reserve 10.0.10.57 --ip 10.0.10.21   # its client, another address
 ```
 
 * **The client** is known by its MAC address, its client identifier