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>

CHANGELOG.md

@@ -8,6 +8,10 @@ All notable changes to Steward are documented here. The format follows
 
 ### Added
 
+- **Reserve another address from a lease.** The Reserve dialog on the
+  Leases page fills in the lease's address but lets you change it, to
+  any address of the same subnet (`steward lease reserve IP --ip ADDR`,
+  `"ip"` in the API). The client moves there when it next renews.
 - **Failed logins in the journal.** A failed login and a refused API
   token are logged as well as audited, with the address first
   (`msg="login failed" ip=… user=…`, `msg="token refused" ip=…`), for