Build a modal dialog

• Renders into a portal at the end of document.body with a backdrop; controlled by an open prop and onClose
• Escape and a click on the backdrop call onClose; clicks inside the dialog do not
• Focus moves into the dialog on open, is trapped (Tab and Shift+Tab cycle inside), and returns to the element that opened it on close
• Background scrolling is locked while open and restored on close, without layout shift from the disappearing scrollbar
• Uses role="dialog" (or the native dialog element), aria-modal="true", and aria-labelledby pointing at the title
• Content behind the dialog is inert to screen readers and keyboard (use inert or aria-hidden on the app root)
• Supports nested or stacked modals: Escape closes only the top one

Only after every requirement works.

• Enter and exit animation that respects reduced motion
• A confirm-dialog variant with a promise-returning confirm() helper
• Close button and optional initialFocus ref