feat(search): reject invalid GitHub usernames before searching

[hyuraku]

概要

Add unit tests with vitest, and stop the app from sending a request when the typed name cannot be a GitHub username.

変更点

• Add vitest, a test / test:watch script, and run yarn test in the Node.js CI workflow
• Pin all vite copies to one range with a resolutions entry so vitest and vite do not install two versions
• Add src/utils/validateUsername.ts with the GitHub username rules, covered by 20 unit tests
• SearchBar shows an inline error instead of searching when the name is invalid. The error only appears on submit, and clears as soon as the name changes
• The error is wired up for screen readers with aria-invalid, aria-describedby and role="alert"
• Add a Cypress spec for the validation flow

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