Improve UX for adding images to a gallery block from the media library

[dhruvikpatel18]

What?

Closes #48417

Improves the UX for adding images to a Gallery block when there is only a single image.
This PR ensures that a placeholder (empty image block) is always available after inserting the first image, making it easier to continue adding more without extra clicks.

Why?

Currently, when a Gallery block has only one image, it is not obvious how to add additional images from the media library.
Users must:

1. Select the existing image,
2. Select the gallery via the small toolbar icon,
3. Click "Add",
4. Open the media library,
5. Select more images,
6. Click "Add to gallery",
7. Finally, click "Update gallery".

This creates unnecessary friction. The “+” appender only becomes visible once the gallery has 2 or more images.
This PR reduces that friction by always showing an empty Image block placeholder immediately after the first image is added.

Testing Instructions

1. Create a new post or page.
2. Insert a Gallery block.
3. Add a single image from the media library.
   • Expected: An empty image placeholder is automatically appended after the first image.
4. Add additional images — the appender and placeholder should behave as before.
5. Remove images back down to one — the placeholder remains available to add another.
6. Confirm drag-and-drop still works as expected.

Screenshots or screencast

Gallery.Block.mov

[im3dabasia]

This does seem like a good addition, similar to the Buttons block, and other design-related blocks such as Columns, Grid, and Group, which have an appender. The gallery block currently lacks one. so adding it would improve consistency.

If there’s enough consensus, should we raise a separate follow-up PR to address this?

[dhruvikpatel18]

Thanks for the suggestion, I have raised separate follow-up PR to address above: #71236

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Co-authored-by: dhruvikpatel18 <dhruvik18@git.wordpress.org>
Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>
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[jasmussen]

Thanks for the PR. Left some related feedback here as well.

It's not a bad instinct, but it's not clear to me yet that this is a clear improvement. The main piece on my mind is to improve the media library itself, for which I've seen a few efforts being underway. In this PR, even if I appreciate the motivation and it arguably has a nicer setup state, it seems like it would actually add more clicks to build a gallery, by having to engage with each image placeholder individually rather than just multi selecting images inside the modal.