docs: refine live playground editor chrome

[wyf027]

Summary

• Refine the LivePlayground code editor toolbar with a clearer Code title and language pill, closer to mature component docs code panels.
• Compose the docs playground shell with Nova UI Flex, Tag, and Text instead of raw React div/span elements where practical.
• Polish editor toolbar, gutter, and language badge styles while keeping the existing live preview/editor behavior.

Validation

• GitHub Actions CI run 28912833852 passed for commit 566a6f0d603c78354bf458e833a2961e7810fb1d.
• CI quality job passed: Install, Lint UI, Typecheck UI, Test UI, Build UI, Build docs.
• Local screenshot capture and local npm run build could not be executed because the local shell runner failed to create a process in this Codex environment.