Store the person uuid the master issues, so renames keep our identity

The welcome now carries a personId; it lands in the instance_id setting
and rides on every hello and HTTP report from then on. The master keys
the person by that uuid, so changing instance_name moves our history to
the new name instead of starting a stranger. Read-only in Settings on
purpose — editing it by hand would detach this machine from its person.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/settings/page.tsx

@@ -17,6 +17,7 @@ interface AppSettings {
     max_concurrent_accounts: string | null;
     master_dashboard_url: string | null;
     instance_name: string | null;
+    instance_id: string | null;
     trading_hours: string | null;
 }
 
@@ -34,6 +35,7 @@ export default function SettingsPage() {
     } | null>(null);
     const [syncing, setSyncing] = useState(false);
     const [instanceName, setInstanceName] = useState<string>('');
+    const [instanceId, setInstanceId] = useState<string>('');
     const [saving, setSaving] = useState(false);
     const [saved, setSaved] = useState(false);
 
@@ -75,6 +77,9 @@ export default function SettingsPage() {
                 if (s.instance_name != null) {
                     setInstanceName(s.instance_name);
                 }
+                if (s.instance_id != null) {
+                    setInstanceId(s.instance_id);
+                }
             });
 
         // Load cached contracts; if cache is empty, auto-resolve
@@ -211,7 +216,11 @@ export default function SettingsPage() {
                         <div>
                             <p className="text-sm font-medium text-slate-800">Instance Name</p>
                             <p className="text-xs text-slate-400 mt-0.5">
-                                Identifier for this autotrader on the master dashboard
+                                Identifier for this autotrader on the master dashboard.
+                                Renaming is safe — the master tracks this machine by its
+                                person ID{instanceId ? (
+                                    <> (<span className="font-mono">{instanceId.slice(0, 8)}</span>)</>
+                                ) : ' once it has connected'}, so history follows the new name.
                             </p>
                         </div>
                         <input