feat(frontend): improve runners errors ui (#5026)

# Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my feature works
- [ ] New and existing unit tests pass locally with my changes

Author: jog1t

frontend/AGENTS.md

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+CLAUDE.md

frontend/CLAUDE.md

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+# Frontend
+
+- When building new UI components, ship a Ladle story alongside the component (`*.stories.tsx` next to the source). Cover the meaningful states (empty, loading, error, success, edge cases) so the component can be reviewed and iterated on in isolation without booting the whole dashboard. Run with `pnpm dev:ladle` from `frontend/`. Existing example: [src/app/runner-pool-error-popover.stories.tsx](src/app/runner-pool-error-popover.stories.tsx).