ci: migrate coverage to Vitest and improve resource usage display

- Migrate coverage reporting from Bun's native coverage to Vitest with Istanbul provider
- Update CI workflow to use `node -e` syntax instead of heredoc for better compatibility
- Add @vitest/coverage-istanbul and vitest as dev dependencies
- Refactor packageInfo() to use spread operator with DEFAULT_PACKAGE_INFO fallback
- Optimize proxy logging to conditionally capture upstream response body only when needed
- Refactor proxyBodyOptions to reduce conditional nesting and improve readability
- Rename ResourceUsageHeader to StatusHeader with improved layout calculation
- Enhance resource usage display with proportional width allocation and rule separators
- Update resource usage header format to show memory breakdown with heap information
- Bump version to 0.2.0 in package.json