feat: Add vitest configuration and unit tests for index page

[gitoak]

This commit adds the vitest configuration file (vitest.config.ts) and unit tests for the index page (pages/index.spec.ts). The vitest configuration sets up the test environment for Nuxt and specifies the use of JSDOM as the DOM environment. The unit tests for the index page ensure that it renders correctly and contains the expected content.

The following files were added or modified:

• vitest.config.ts
• pages/index.spec.ts
• test/setup.ts
• package.json (added test:unit and test:unit:watch scripts)

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