chore: harden SSR safety and tests

[Tnalxmsk]

Summary

Improve SSR safety and stability for the global audio singleton, add a dispose API, and reorganize tests under a
dedicated test directory.

Related Issues

• Closes chore: stability audit and tests #12

Changes

• Add SSR guards for audio creation and storage access; ensure safe no-op behavior on the server
• Introduce audioManager.dispose() to remove listeners and reset state
• Add SSR + dispose tests, and split manager vs hook tests into test/
• Update README with dispose usage and SSR note
• Add @testing-library/dom and refine Vitest test include path

How To Test

1. npm run test:run
2. npm run lint
3. npm run build

Checklist

• Title follows the convention (e.g., feat: add global audio seek guard)
• Build passes locally (pnpm 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

• SSR mode now no-ops audio actions and keeps the default snapshot state.
• dispose() is optional and intended for tests/cleanup.

Screenshots / Recordings (if UI changes)

N/A