ci: add Vitest unit tests, Playwright E2E browser tests and GitHub Actions CI workflow

[Ming2k8-Coder]

Description

This PR establishes a complete automated testing suite for the repository:

• Unit & DOM Sanity Tests: Vitest + JSDOM testing HTML markup, meta tags, importmaps, and DOM element IDs (tests/unit/sanity.test.js).
• E2E Browser Automation: Playwright tests covering model loading, tab navigation, language switching, ARIA attributes, lighting controls, and VRMA playback (tests/e2e/viewer.spec.js).
• CI/CD Pipeline: GitHub Actions workflow running tests automatically on push and pull requests (.github/workflows/test.yml).

Diff Size

~369 lines added across test files and configs.