Document local verification steps for frontend, backend, and UI changes

## Summary

The repository documents individual commands, but a first-time contributor does not have a quick guide to which checks to run for a documentation, frontend, backend, or UI end-to-end change.

## Goal

Add a concise local-verification section to the contribution guide.

## Scope

- Describe the minimum relevant checks for documentation-only, frontend, and backend changes.
- Include the existing lint, frontend build, Ruff, and pytest commands.
- Briefly explain that the Playwright UI specs need a locally running app and the expected sample data; do not imply they run automatically in CI.
- Keep the section short and link to existing instructions rather than duplicating them.

## Acceptance criteria

- A newcomer can choose the appropriate checks for a small contribution.
- Commands match the scripts and configuration currently in this repository.
- The guide clearly distinguishes regular automated checks from the locally prepared Playwright UI checks.
- No application behavior or CI configuration changes are required.

## Helpful files

- `CONTRIBUTING.md`
- `README.md`
- `frontend/e2e/orbit-ui.spec.ts`
- `.github/workflows/ci.yml`