Reference existing shortcuts inside Macro Sequence steps

Add a "Run existing shortcut" step type to MACRO_STEP_TYPES. When selected, it shows a searchable picker (use the existing 32px / 6px pill aesthetic per memory feedback_ui_consistency_32px) listing all assignments across all profiles. At runtime the step invokes the same action runner as the source assignment. Cheaper than synthesising the key combo because it preserves the action's parameters too.