[cross-collection] Storybook-style playground for overrides

[tsemachh]

Plugin: cross-collection · Priority: P2 · Tracked by ROADMAP.md

Problem

Hard to discover which overrides exist.

Proposed solution

Admin page listing every override + live preview.

Acceptance criteria

• Page reachable in dev.
• Lists active overrides.
• Hidden in production.

How to claim

Comment on this issue (e.g. "I'd like to work on this") and a maintainer will assign it to you. Open a PR referencing this issue (Closes #<n>). Discuss design questions here or on the Payload Discord #plugins channel.