Fix security vulnerabilities: upgrade vitest to 4.0.16

- Resolves 5 moderate severity vulnerabilities in esbuild/vite chain
- Updates vitest from 2.x to 4.0.16
- Updates @vitest/coverage-v8 to match
- All 118 tests passing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: karlwaldman