Toolbox keyboard: ArrowRight focuses flyout; Enter/Space toggle categories

[tracygardner]

Motivation

• Improve keyboard navigation so pressing ArrowRight on a toolbox category always moves focus into the flyout when the flyout is open.
• Make Enter/Space open and close collapsible toolbox categories instead of using lateral arrow keys for that action.

Description

• Updated main/blocklyinit.js to add a wireToolboxKeyboardOverrides IIFE that installs a keydown listener on the toolbox DOM (toolbox.HtmlDiv or .blocklyToolboxDiv).
• The listener ignores text inputs and content-editable elements and intercepts ArrowRight to focus the flyout workspace when flyout.isVisible() is true using Blockly.getFocusManager().focusTree(flyoutWorkspace).
• The listener also maps Enter / Space / Spacebar to toggle expandable toolbox categories by calling selectedItem.toggleExpanded() when available.
• Existing monkey-patch that no-ops toolbox.onKeyDown_ remains to defer core handling to the keyboard-navigation plugin.

Testing

• No automated tests were run for this change.

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Codex Task