Product Details: Display cover tag on the first product image

[itsmeichigo]

Closes: #15039

Description

This PR addresses a confusion among users about cover images of products. The solution is to simply display a label "Cover" on the first image of both the product form and the product image list screens. The helper text on the product image list screen also mentions the cover text being set for the first image to make this clearer.

Steps to reproduce

• Log in to a test store.
• Open an existing product or create a new one.
• Upload one or more images for the product as needed.
• Confirm that the first image listed on the product form has the "Cover" label while the rest don't.
• Confirm that the label looks good in both light and dark modes.
• Select any image to open the image list screen.
• Confirm that the first image on this screen also displays the "Cover" label.
• Confirm that the helper text on this screen includes the detail about the cover image.
• Drag and drop images and confirm that the "Cover" label is updated correctly for the images.

Testing information

Tested on simulator iPhone 16 Pro iOS 18.2 and confirmed that the "Cover" label looks good in both dark and light modes and in different text sizes. The label is updated correctly when the images positions are updated.

Screenshots

Screen	Light mode	Dark mode
Product form
Photos

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

TODO: @itsmeichigo to update the release notes for 21.7 after the code freeze of 21.6 is done.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS
	Build Number	pr15041-7439764
	Version	21.6
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	7439764
	App Center Build	WooCommerce - Prototype Builds #12780

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[pmusolino]

I tested in white/dark mode and also with larget text. Everything works like a charm.