Add onboarding wizard and revamp agent creation flow

Add OnboardingWizard component for first-time company setup. Rework
NewAgentDialog into a multi-step wizard with adapter selection and
config. Add server route for agent connection string generation.
Wire onboarding into Dashboard and Layout. Update DialogContext with
onboarding state.

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

ui/src/pages/Agents.tsx

@@ -118,7 +118,7 @@ export function Agents() {
       {agents && agents.length === 0 && (
         <EmptyState
           icon={Bot}
-          message="No agents yet."
+          message="Create your first agent to get started."
           action="New Agent"
           onAction={openNewAgent}
         />