Add opacity keyboard shortcuts matching Figma: pressing Digit1-Digit9
sets the selected layers to 10%-90% opacity, and Digit0 sets 100%.
- Add 'selection.setOpacity' EditorCommandId with registry metadata
(shortcut display '1-9, 0') so it appears in the command cheatsheet
- Implement the command in selection.ts using editor.undo.runBatch +
updateNodeWithUndo, following the toggleVisibility/setBlendMode pattern
- Add setOpacityTarget closure in useEditorCommands so the external
keyboard handler can inject the digit value before runCommand
- Add canSetOpacity capability (enabled when there is a selection)
- Add bindOpacityKeys handler in src/app/shell/keyboard/opacity.ts,
following the nudging.ts external-handler pattern since the opacity
value comes from event.code and cannot be expressed as a fixed
keybinding in the registry
- Guard against text editing, number field focus, and modifier keys
- Add i18n label 'setOpacity' across all 8 locales
- Add unit tests for single/multi selection, undo batch, and no-op
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