feat: add support to invoke an apple shortcut

[ericfitz]

For app configuration we currently accept a shortcut key sequence.

This feature is composed of:

• per-app-shortcut, add the ability to choose either a key sequence (default) or shortcut invocation
• in settings ui, add the ability to select a shortcut if the user chooses to map a shortcut instead of a key sequence
• add the code to invoke the shortcut if so configured and button is pressed

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Design spec: docs/superpowers/specs/2026-06-03-apple-shortcut-action-design.md