Add testing framework (Vitest)

[needsbuilder]

Summary

EasyClaw currently has no test framework. Adding Vitest would improve code quality and enable CI test coverage.

Suggested Scope

1. Install Vitest as a dev dependency
2. Add npm run test script to package.json
3. Write a few initial unit tests for utility functions (e.g., src/main/services/path-utils.ts, src/main/services/env-checker.ts)
4. Add test step to the CI workflow (.github/workflows/ci.yml)

Why This Is a Good First Issue

• Clear scope with well-defined steps
• No deep knowledge of the Electron main process required for utility tests
• Vitest has excellent documentation