Add explicit return type annotation to cn function

[Lang-Qiu]

Background

The cn function in src/lib/utils.ts lacked an explicit return type annotation. Although TypeScript can infer the type, adding an explicit annotation improves code readability and maintainability, which aligns with TypeScript best practices for type safety and documentation.

Changes

• Added explicit return type : string to the cn function definition in src/lib/utils.ts.

Verification

• Run the TypeScript compiler (tsc) to ensure no compilation errors.
• Verify existing tests pass to confirm no functional changes, as this is a type-only update that preserves behavior.