[UEPR-483] "Choose a Sprite" and "Choose a Backdrop" keyboard accessibility

[kbangelov]

Resolves

https://scratchfoundation.atlassian.net/browse/UEPR-483

Proposed Changes

• Adds keyboard navigation logic.
• UI bug: tooltips don't refresh in case of first open or shift+tab-ing back to the menu.

Reason for Changes

Part of accessibility initiative for Scratch.

[Copilot]

Pull request overview

Improves keyboard accessibility for the “Choose a Sprite” / “Choose a Backdrop” action menus in Scratch GUI as part of the accessibility initiative (UEPR-483).

Changes:

• Refactors ActionMenu from a class component to a hook-based functional component and adds keyboard navigation/focus handling.
• Updates ActionMenu button styling to show focus state similar to hover.
• Minor test/comment and formatting cleanup.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
packages/scratch-gui/test/integration/sounds.test.js	Adds a clarifying comment for the keyboard shortcuts integration test.
packages/scratch-gui/src/components/stage-selector/stage-selector.jsx	Removes an extraneous blank line in the moreButtons config.
packages/scratch-gui/src/components/action-menu/action-menu.jsx	Implements keyboard navigation + refactors ActionMenu to hooks/memoization.
packages/scratch-gui/src/components/action-menu/action-menu.css	Adds :focus styling to match hover for action menu buttons.

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[adzhindzhi]

Looks good to me! 👍