Fix: Prevent visual jitter when changing block styles

[DarkMatter-999]

What?

Closes #77830

This PR updates the BlockProps hook in the block editor to use useLayoutEffect instead of useEffect when setting wrapper properties.

Why?

When a block style variation (like "Rounded") is applied, there is currently a visual "jerk" or flash. This happens because useEffect runs after the browser has already painted the initial state.

In the case of the Image block (e.g., in Twenty Twenty-Four theme):

1. The browser paints the default "Rounded" style (which defaults to a 9999px border-radius/circle).
2. useEffect fires, adding the specific theme variation classes (e.g., is-style-rounded-[hash]).
3. The browser paints again with the theme's specific border-radius (e.g., 20px), causing a visible snap from a circle to a rounded square.

How?

By switching to useLayoutEffect, the wrapper properties (including theme-specific variation classes) are applied synchronously after all DOM mutations but before the browser has a chance to paint. This ensures the very first frame the user sees already contains the correct, theme-defined CSS classes, eliminating the intermediate "fully rounded" state.

Testing Instructions

1. Use a theme that defines a specific border-radius for the "Rounded" image variation (e.g., Twenty Twenty-Four).
2. Open the Site Editor or Post Editor.
3. Insert an Image block and select an image.
4. In the Settings Sidebar, click on the Styles tab.
5. Switch the style from "Default" to "Rounded".
6. Observe: The image should transition to the theme's rounded setting immediately without flashing as a full circle first.

Screenshots or screencast

Before

block-style-jitter.mov

After

block-style-jitter-fix.mov

Use of AI Tools

I used Gemini 3 Flash to help draft PR description.

[github-actions]

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