chore(deps): upgrade build tooling to clear audit vulnerabilities

Bump vite 5->8, vitest 2->4, and @vitejs/plugin-react 4->6 (all dev-only). Resolves all 5 Dependabot advisories (1 critical, 1 high, 3 moderate), which were confined to the dev server and test runner. Typecheck, build, and all 37 tests pass on the new majors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dpsiderius