Add image upload support for product variants

[chahmedejaz]

What? Why?

• Closes As an enterprise user, I want to upload an image for a variant #13978

Enterprise users can now upload images for individual product variants from the admin products page. Previously, only product-level images were supported — variant rows showed an empty column with no placeholder, and the image upload overlay only handled products.

This change:

• Adds a has_one :image association to Spree::Variant, reusing the existing Spree::Image model (which was already polymorphic via viewable_type/viewable_id).
• Updates the images controller to accept a variant_id param, setting the correct viewable_type (Spree::Variant vs Spree::Product) and redirecting back to the admin products list after save.
• Replaces the _product_image partial with a generic _image partial that works for both products and variants.
• Adds a placeholder image (40% opacity, transitioning to full opacity on row hover) for variants without an image.
• Updates the edit-modal title to "Edit image for {product name}" for products and "Edit image for {product name} - {variant name}" for variants, with an explanatory subtitle recommending 440×440px minimum.
• Includes an irreversible data migration to clean up any orphan variant images that may exist from before this feature.

What should we test?

1. Placeholder image for variants without an image

   • Navigate to /admin/products.
   • Verify each variant row shows a default placeholder image at 40% opacity.
   • Hover over a variant row and confirm the placeholder transitions to full opacity.

2. Upload an image for a variant

   • Click the "Edit" button on a variant's placeholder image.
   • Confirm the modal title reads "Edit image for {product name} - {variant name}".
   • Confirm the subtitle "A square size starting from 440px by 440px is recommended." is visible.
   • Attach a valid image file and save.
   • Verify the success flash message and that the variant row now shows the uploaded thumbnail.

3. Upload an invalid file type

   • Open the edit modal for a variant.
   • Attach a non-image file (e.g. PDF or CSV).
   • Confirm an error message is displayed within the modal.

4. Update an existing variant image

   • Upload a second image for a variant that already has one.
   • Verify the thumbnail updates and the success flash message appears.

5. Product image upload still works

   • Click the "Edit" button on a product image.
   • Confirm the modal title reads "Edit image for {product name}" (no variant suffix).
   • Upload a new product image and confirm it saves correctly.

6. Readonly product/variant image displays without edit button

   • Set up a producer relationship granting only create_linked_variants permission (not manage_products) on another producer's variant.
   • Navigate to /admin/products.
   • Locate the readonly product row - verify the image thumbnail is present but there is no "Edit" button on the image.
   • Locate the readonly variant row - verify the image thumbnail is present but there is no "Edit" button on the image.

7. Variant image redirects to product list

   • After uploading or updating a variant image, confirm the browser redirects to the admin products list.

8. Migration — existing variant images are removed

   • If the environment has any pre-existing Spree::Image records with viewable_type = 'Spree::Variant', verify they are cleaned up after running db:migrate. Product images are unaffected.
   • This will be tested by a dev because right now we don't show existing variant images anywhere.

9. Any other scenario mentioned in the issue and regression testing of the product page.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• User facing changes
• API changes
• Technical changes only
• Feature toggled