fix: use the declared class name utility

[lindixu6-hash]

Problem

Issue #22 reports that generated components crash when they render Typography because it imports classnames, which is not declared by the @react-agent/shadcn-ui workspace.

The same undeclared direct import also exists in the exported JsonSkeleton example. classnames appears in yarn.lock only as a transitive dependency, so relying on it is not a stable workspace contract.

Change

Use the design system's existing cn helper in both locations:

• components/ui/typography.tsx
• components/examples/json-skeleton/demo.tsx

That helper is already used throughout the component library and is backed by the workspace's declared clsx and tailwind-merge dependencies. No dependency or lockfile changes are needed.

Closes #22.

Verification

• both changed TSX files transpile without TypeScript syntax diagnostics
• both relative cn imports resolve to frontend/shadcn-ui/src/lib/utils.ts
• clsx and tailwind-merge are direct workspace dependencies
• no classnames imports remain under frontend/shadcn-ui/src
• git diff --check passes

I could not claim a full workspace build: two clean Yarn v1 install attempts remained in the dependency-fetch stage, and an offline retry identified an incomplete cached package. This PR therefore reports the deterministic source-level checks above rather than presenting a partial install as a green build.