Assistant: drop a2ui surfaces, fall back to markdown tables

CK 1.60+ validates generated a2ui ops against a component catalog before
painting, and our agent's ops fail validation — surfaces hang at
"Building interface" forever (MAF also drops RunAgentInput.context, so
the catalog/guidelines never even reach the model). Disable a2ui end to
end until the pipeline matures:

- server.ts: a2ui { enabled: false }; RENDER_TOOLS no longer includes
  render_a2ui
- App.tsx: remove the a2ui provider prop
- agent.py: prompt now asks for clean markdown tables for lists and
  structured data; render_spending_summary card guidance unchanged

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

frontend/src/App.tsx

@@ -77,7 +77,7 @@ export default function App() {
         <PrivacyProvider>
           <AuthProvider>
             <QueryClientProvider client={queryClient}>
-              <CopilotKit runtimeUrl="/api/copilotkit" agent="wealthysmart" a2ui={{}}>
+              <CopilotKit runtimeUrl="/api/copilotkit" agent="wealthysmart">
                 <AppRoutes />
                 <Toaster richColors position="top-right" closeButton />
               </CopilotKit>