Replace custom widgetRenderer with CopilotKit Open Generative UI

Start with apps/app/src/app/api/copilotkit/route.ts and verify whether openGenerativeUI supports the current LangGraphHttpAgent runtime; compare this with the linked CopilotKit documentation. Then inspect use-generative-ui-examples.tsx, widget-renderer.tsx, idiomorph-inline.ts, export-overlay.tsx, and export-utils.ts, including the CopilotKit provider's sandboxFunctions. Done means generated calculator and binary-search UIs work, theme toggling remains functional, custom helpers are removed, and README/CLAUDE.md no longer reference widgetRenderer.

Background

We currently ship a hand-rolled open generative UI implementation:

• WidgetRenderer — ~730-line component that renders agent-authored HTML/CSS/JS inside a sandboxed iframe, with our own theme CSS, idiomorph streaming, and export overlay.
• Registered as a useComponent({ name: "widgetRenderer", ... }) in use-generative-ui-examples.tsx.

CopilotKit now ships this as a first-class feature: Open Generative UI. It covers the same use case (live-streamed sandboxed HTML/CSS/JS, CDN libraries, two-way host bridges) without per-app widget code.

Goal

Delete the custom widget pipeline and use CopilotKit's built-in Open Generative UI instead.

Scope

• Enable openGenerativeUI: true in the runtime (route.ts).
• Remove the widgetRenderer useComponent registration and its supporting files (widget-renderer.tsx, idiomorph-inline.ts, export-overlay.tsx, export-utils.ts) once parity is confirmed.
• Port any host-side hooks we relied on (e.g. theme toggling) to sandboxFunctions on the CopilotKit provider.
• Keep the other declarative components (pieChart, barChart, plan_visualization, HITL scheduleTime) as-is.
• Update CLAUDE.md and any demo prompts/suggestions referencing "widgetRenderer".

Open question

The current runtime is v1 CopilotRuntime + LangGraphHttpAgent pointing at the Python LangGraph agent. The docs example for openGenerativeUI: true uses the v2 runtime with BuiltInAgent + InMemoryAgentRunner. We need to verify whether the flag is supported alongside LangGraphHttpAgent, or whether this upgrade also requires migrating the runtime wiring (and what that means for the Python agent in apps/agent/).

Acceptance criteria

• "Build me a calculator" / "Visualize binary search" prompts produce a streamed, sandboxed UI in chat with no custom widget code in this repo.
• Theme toggle still works from generated UI (via sandboxFunctions).
• Custom widget-renderer.tsx and helpers are deleted.
• README/CLAUDE.md reflect the new approach.

Docs: https://docs.copilotkit.ai/generative-ui/open-generative-ui