Upgrade outdated devDependencies

[bntvllnt]

Problem

Three devDependencies are 1-3 major versions behind:

• @types/node: 22.19.15 → 25.x (3 major versions)
• eslint: 9.39.4 → 10.x (1 major version)
• vitest: 3.2.4 → 4.x (1 major version)

Risk

Low (devDeps only), but eslint 10+ and vitest 4 may have breaking API changes that affect CI.

Acceptance Criteria

• Bump all three devDeps
• Run full test suite (33 tests) against new versions
• Fix any breaking changes in test/lint config
• Verify CI passes

Priority

Low — no runtime impact, maintenance hygiene.