feat: add center morph modal

[starc007]

Summary

• add a composable center morph modal that unfolds outward from a compact center seed
• add the beUI Pro preview, registry metadata, accessibility coverage, and component tests
• make Morph Popover closing follow the same motion path as opening

Why

The modal needs to feel like one continuous surface emerging from the center rather than layered panels appearing in sequence. The Morph Popover exit also used a separate choreography, which made closing feel disconnected and abrupt compared with opening.

Validation

• bunx biome check components/motion/popover-morph.tsx
• bun run typecheck
• bun test
• bun run check:registry
• git diff --check