DropZone: Make the drop zone in Storybook the same size as the item

[t-hamano]

What?

By adding position:relative to the parent element, make the size of the drop zone component match the size of the item.

At the same time, increase the padding a bit because it was a bit cramped when we were about to drop it.

Testing Instructions

Access http://localhost:50240/?path=/story/components-dropzone--default

Screenshots or screencast

Before

before.mp4

After

after.mp4

[github-actions]

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Co-authored-by: t-hamano <[email protected]>
Co-authored-by: tyxla <[email protected]>

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[github-actions]

Flaky tests detected in 92cb3a3.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/12467023853
📝 Reported issues:

• [Flaky Test] Navigates the items list via UP/DOWN arrow keys #64334 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js

[tyxla]

Makes sense to me 👍