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/cli.md

@@ -100,7 +100,7 @@ See [subnets.md](subnets.md#access-allow-and-deny-lists).
 |---|---|
 | `lease list [--subnet S] [--state STATE] [--interface I] [--search TEXT] [-a]` | active and held-back leases; `-a` all |
 | `lease show IP` | one lease |
-| `lease reserve IP [--hostname H] [--by mac\|client-id\|both]` | make the address its client's reservation, by its MAC address and client ID (either matches) unless `--by` says one |
+| `lease reserve IP [--ip ADDR] [--hostname H] [--by mac\|client-id\|both]` | make the address its client's reservation, by its MAC address and client ID (either matches) unless `--by` says one; `--ip` reserves another address of the same subnet instead |
 | `lease delete IP [-y]` | free the address at once |
 
 ### activity