test: #10 set up vitest and add initial audio tests

[Tnalxmsk]

Summary

Set up Vitest-based testing infrastructure and add initial unit tests for the audio manager and hook.

Related Issues

• Closes test: set up Vitest testing infrastructure #10

Changes

We needed a lightweight test runner to validate the global singleton audio manager and the React hook behavior without
introducing heavy setup or browser-based E2E tooling. Since this package is distributed as an ESM + TypeScript library,
the test tool needed to align with that environment and keep configuration overhead low.

Vitest was chosen because it is ESM‑friendly out of the box, integrates cleanly with TypeScript, and provides fast
feedback for library development. It also works well with @testing-library/react for hook tests and supports jsdom, which
lets us mock Audio/DOM APIs and test our logic without real media files or a full browser.

In short, Vitest gives us the simplest path to reliable unit/hook tests while matching the project’s ESM/TS setup and
keeping maintenance cost small.

• Adopt Vitest as the test runner because this repo is ESM (type: "module") and TypeScript-based, and Vitest fits that
  setup with minimal configuration and fast feedback for library development.
• Configure a jsdom environment and a dedicated setup file so DOM- and Audio-related APIs work reliably in tests without
  real media files or a browser.
• Mock HTMLMediaElement methods (play, pause, load) and ensure an Audio constructor exists in jsdom, which keeps Audio-
  dependent tests stable and prevents runtime errors.
• Add initial tests for audioManager to cover core behaviors (source reset, clamped volume/rate, pending seek after
  metadata, error state, stop behavior, event fanout).
• Add initial tests for useGlobalAudio to validate shared singleton state across hooks and autoPlay wiring, plus add test
  scripts and dev dependencies.

How To Test

npm run test

Checklist

• Title follows the convention (e.g., feat: add global audio seek guard)
• Build passes locally (npm run build)
• No breaking changes, or they are documented below
• Tests added/updated where appropriate
• Docs/comments updated where helpful

Breaking Changes (if any)

None.

Notes For Reviewers

None

Screenshots / Recordings (if UI changes)

N/A