feat(shared): add Modal component

Native <dialog>+showModal() wrapper with imperative open()/close() via ref,
body scroll lock, and backdrop-click/keyboard-close behaviors. Exposes
onCancel and onBackdropClose escape hatches for consumers that need to
wrap the close path (e.g. in a view transition).

src/shared/ui/index.ts

@@ -9,6 +9,7 @@ export { InlineSvg } from './InlineSvg';
 export { Input, Textarea } from './Input';
 export type { LinkVariant } from './Link';
 export { Link } from './Link';
+export { Modal, type ModalHandle } from './Modal';
 export { RichText } from './RichText';
 export type { ContainerSize, SectionBackground } from './Section';
 export { Container, Section } from './Section';