fix(electron): auto-navigate to home after keygen completion

Previously, after keygen completed, the Session page would just update the
status to 'completed' but not navigate away. Users had to manually click
the "Return Home" button. This could result in a white screen if the button
wasn't visible or clickable.

Now the page auto-navigates to home after 2 seconds, giving users time to
see the completion status and public key before redirecting.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-app/src/pages/Session.tsx

@@ -70,6 +70,10 @@ export default function Session() {
             status: 'completed',
             publicKey: event.publicKey,
           } : null);
+          // Auto-navigate to home after a short delay to show completion status
+          setTimeout(() => {
+            navigate('/');
+          }, 2000);
         } else if (event.type === 'failed') {
           setSession(prev => prev ? {
             ...prev,