InputVariantsinterface

Styling variants shared by every form input — currently the width variant (width="narrow", "normal", "wide", "full", "fit").

• Extends WidthVariants so any input can be sized (e.g. <CheckboxInput width="fit"> to shrink to its content).
• Designed to grow: new cross-cutting input styling props (e.g. spacing) should be added here so every input picks them up consistently.