ui: show no-agents banner on dashboard with link to onboarding step 2

When a company has no agents (e.g. user exited onboarding early), the
dashboard now shows an amber banner: "You have no agents. Create one
here" that opens the onboarding wizard directly at step 2 (agent
creation), skipping the company creation step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/App.tsx

@@ -186,7 +186,7 @@ function NoCompaniesStartPage() {
           Get started by creating a company.
         </p>
         <div className="mt-4">
-          <Button onClick={openOnboarding}>New Company</Button>
+          <Button onClick={() => openOnboarding()}>New Company</Button>
         </div>
       </div>
       <OnboardingWizard />