fix(ui): restore stack animations and shadow tokens

[yyyyaaa]

Summary

• restore the first Stack card's enter animation and apply configured enter and exit tweens before settled cards return to spring-based offset motion
• make Stack entry and exit instant when reduced motion is requested, with focused regression coverage for transition configuration and accessibility behavior
• restore visible light and dark elevation shadow tokens, then synchronize the generated package CSS and registry theme
• build all workspace package outputs before Blocks development starts so fresh worktrees can resolve package runtime and CSS exports

Verification

• pnpm check
• pnpm build:pages
• pnpm pack:local
• pnpm dev
• verified GET http://127.0.0.1:3005/ returns HTTP 200