Background: Fix the legacy gradient UI where a gradient cannot be selected - #81056

What?

Follow up to #77279

When a theme opts out of the new gradient background in theme.json, the Background panel falls back to the legacy gradient UI. I noticed that selecting a preset there does nothing at all, so this PR fixes it.

Why?

The Background panel and the block inspector hook that saves the value were deciding "which gradient is this?" by two different rules.

• The panel looks at the settings.background.gradient value resolved from theme.json. When a theme sets it to false, the panel hides the new gradient control and renders the legacy one instead, which writes to style.color.gradient.
• The hook looked only at whether the block declares the background.gradient block support. core/group declares it, so the hook always expected the value at style.background.gradient.

With the setting disabled, the panel wrote to one place and the hook read from another. Finding nothing, the hook saved an empty value, and the gradient never made it onto the block.

How?

Make the hook use the same rule as the panel: the block support and the resolved theme.json setting.

Testing Instructions

1. Add the background.gradient opt-out to your theme's theme.json:

{
	"$schema": "../../schemas/json/theme.json",
	"version": 3,
	"settings": {
		"appearanceTools": true,
		"layout": {
			"contentSize": "840px",
			"wideSize": "1100px"
		},
		"background": {
			"gradient": false
		}
	}
}

2. Insert a Group block and open the Background panel in the inspector.
3. Open the Gradient control and pick a preset.
4. The gradient should be applied to the block, and it should persist after saving and reloading.

Use of AI Tools

Claude Code was used to locate the cause, write the fix and author the unit tests. All changes were reviewed by me.