chore(deps): upgrade vitest to v4.0.15

Update vitest and @vitest/coverage-v8 from v3.2.4 to v4.0.15.

Vitest v4 introduces several improvements including:
- Stable browser mode
- Visual regression testing support
- Rewritten pool architecture without Tinypool
- Workspace renamed to projects configuration

This upgrade prepares the codebase for using v4's projects feature
for monorepo test aggregation.

Closes #186

Author: ryoppippi