fix(image-web): use icon size in structure preview

[valczak-cell]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

The Image widget applies image dimension logic to icon sources in Studio Pro's Structure mode. When height is set to automatic, the default maxHeight of 250px is used for the placeholder, causing small icons to occupy 250px vertically.

This change preserves the existing placeholder but uses the configured iconSize for both its width and height when the data source is set to Icon.

Design mode and runtime rendering are unaffected.

Fixes #2373

What should be covered while testing?

1. Add an Image widget to a page.
2. Set its data source to Icon.
3. Select an icon and set Icon size to 15px.
4. Open the page in Structure mode.
5. Verify that the placeholder is approximately 15×15px instead of 250px high.
6. Verify that Design mode and runtime rendering remain unchanged.

[leonardomendix]

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