feat(playground): display json-difference version in header

[lukascivil]

Objective

Surface the installed json-difference version directly in the playground UI so visitors can tell which library build they are exercising. Also tighten the playground's Nx/Vite configuration so local dev and nx build behave consistently.

What was done

• Read the library version at build time in apps/playground/src/App.tsx via import { version as jsonDifferenceVersion } from 'json-difference/package.json' and render it as v{version} below the main heading.
• Wrapped the heading and version text in a centered Box so the subtitle inherits the layout instead of re-declaring alignment.
• In apps/playground/project.json, declared outputs: ["{options.outputPath}"] on build, switched outputPath to {projectRoot}/dist, wired serve to buildTarget: playground:build, and dropped the now-redundant defaultConfiguration entries.
• In apps/playground/vite.config.ts, pinned server/preview to localhost:3000 and allowed the workspace root via searchForWorkspaceRoot(import.meta.dirname) so Vite can resolve json-difference/package.json from the monorepo.

Test plan

• pnpm nx serve playground and confirm the header shows v<current json-difference version>.
• pnpm nx build playground completes and emits to apps/playground/dist.
• pnpm nx preview playground serves the built app on localhost:3000.