feat(react): redesign skeleton animation api

[wangdicoder]

Summary

• redesign Skeleton into a richer loading primitive with structured layouts and subcomponents
• replace the old ConfigProvider shimmer setting with skeleton.animation and update related docs and demos
• add a major changeset for the react and tokens packages

Release

• major
• affected packages: react, tokens

Test plan

• pnpm --filter @tiny-design/react test -- skeleton config-provider
• pnpm --filter @tiny-design/react exec tsc -p tsconfig.json --noEmit
• verified Skeleton docs page in browser, including dark mode