Add dashboard screenshot refresh guidance

[lpc0387]

3. Add dashboard screenshot refresh guidance

Labels: good first issue, documentation, dashboard, assets

Body:

The README displays docs/screenshots/dashboard.png, but contributors do not yet have a short checklist for refreshing screenshots safely. Add documentation that explains when to update the screenshot, how to use the sandbox rather than production data, and what details to inspect before committing an image.

Suggested scope:

• Add a small section to CONTRIBUTING.md or a launch/docs page.
• Point contributors to the sandbox quick start.
• Require sanitized data and no visible secrets, usernames, private paths, or real logs.
• Keep image replacement outside the issue unless the current screenshot is intentionally being refreshed.

Acceptance criteria:

• The documentation names docs/screenshots/dashboard.png as the canonical dashboard image used by the README.
• The guidance tells contributors to capture screenshots from sandbox or sanitized data only.
• The guidance includes a short pre-commit review checklist for images.
• Verification notes include manual Markdown review; no full test suite is required.

Why it is newcomer-friendly:

This is a bounded contributor-experience improvement that helps future visual changes without requiring frontend or backend code changes.

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This issue was created from the v5.0 launch backlog in docs/launch/good-first-issues.md.