Good first issue: reduce the frontend bundle size

## Summary

The production frontend build succeeds, but Vite reports that the main JavaScript bundle is larger than 500 kB after minification.

```text
(!) Some chunks are larger than 500 kB after minification.
```

This is a good first issue for contributors who want to learn how the React/Vite frontend is organized.

## Goal

Reduce the initial JavaScript bundle size by splitting code that is not needed for the first screen.

## Suggested starting points

- Inspect the Vite build output with a bundle visualizer or Vite's chunk reporting.
- Consider lazy-loading page-level features such as the evaluation, improvement, asset-editor, or chat UI.
- Consider separating large editor or visualization dependencies into dedicated chunks.
- Keep the existing navigation and loading behavior usable while a lazily loaded page is fetched.

## Acceptance criteria

- `pnpm --filter agent-improvement-console-ui run build` succeeds.
- The initial bundle is materially smaller, or large dependencies are placed in clearly separated lazy-loaded chunks.
- Navigation still works for every page.
- No new console errors are introduced.
- Add a short note to the PR explaining the chosen split points and the before/after build output.

## Helpful commands

```bash
pnpm --filter agent-improvement-console-ui run build
pnpm --filter agent-improvement-console-ui run lint
```

Questions are welcome—please comment on this issue before making a large architectural change.