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

---

## 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

packages/admin/dashboard/src/i18n/translations/en.json

@@ -530,8 +530,8 @@
       "successToast": "Media was successfully updated.",
       "variantImages": "Variant images",
       "showAvailableImages": "Show available images",
-      "availableImages": "Available images",
-      "selectToAdd": "Select to add to variant",
+      "availableImages": "Select images",
+      "selectToAdd": "Add product images to the variant. To add new images, add them to the product first.",
       "removeSelected": "Remove Selected"
     },
     "variantMedia": {