feat: add liquid morphing reorderable tabs

[starc007]

What changed

• add a new liquid morphing tabs block with equal-size tabs merged into the content surface
• support click selection and pointer/keyboard reordering
• rebuild drag behavior around fixed slots and spring physics for symmetric, interruptible movement
• move adjacent tabs at the 50% overlap threshold and keep the active liquid surface attached during reordering
• add the public preview, registry metadata, launch date, and install wiring
• upgrade the site to Next.js 16.3 and migrate middleware.ts to the proxy.ts convention

Why

The block provides a polished, reusable tab interaction where the selected tab feels physically joined to its panel while tabs can be reordered without text drift, detached corners, or asymmetric drag jitter.

Validation

• bun run check
• bunx next typegen
• bun test — 174 passing tests
• repeated browser drag checks in both directions, selection after reorder, 50% crossover behavior, and selected-surface alignment