feat(dashboard): variant images list thumbnail + refactor form state management (#13905)

## Summary

**What** — What changes are introduced in this PR?

- show thumbnail on the product variant list
- refactor variant image editor state management
- await revalidation before rendering form

**Testing** — How have these changes been tested, or how can the reviewer test the feature?

Manual testing

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## Checklist

Please ensure the following before requesting a review:

- [x] I have added a **changeset** for this PR
    - Every non-breaking change should be marked as a **patch**
    - To add a changeset, run `yarn changeset` and follow the prompts
- [ ] The changes are covered by relevant **tests**
- [x] I have verified the code works as intended locally
- [ ] I have linked the related issue(s) if applicable

.changeset/serious-doors-pump.md

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+"@medusajs/dashboard": patch
+---
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+feat(dashboard): variant images list thumbnail + refactor form state management