test(error-modal): add comprehensive unit tests for ErrorModal (Vitest + RTL)

[VetteIT]

Description

Add a focused suite of unit tests for ErrorModal covering modal wiring, i18n rendering, external link attributes, refresh behavior, and basic a11y attributes. Tests are written with Vitest + @testing-library/react and follow the style used in recent PRs.

Closes #113

What change does this PR introduce?

• Test coverage for ErrorModal
• Bug fix
• New feature
• Breaking change
• CI/CD
• Documentation update
• Chore

Proposed approach

• Mock react-i18next to return i18n keys as fallback text.
• Mock useGlobalModal to assert setPromptError(null) is called on close.
• Mock MUI Modal to inspect the props passed by the component.
• Verify:
  • open and aria-labelledby are correctly set.
  • Title, error message, and InfoIcon render.
  • Discord link uses the URL from Endpoints and has target="_blank" + rel="noopener noreferrer".
  • window.location.reload is invoked from the Refresh button when the jsdom environment supports spying; otherwise the click is asserted to be safe (no throw).
  • The heading id matches aria-labelledby for basic a11y.

Checklist

• Commit message follows Conventional Commits
• Tests added/updated for the new cases
• No app logic changed; test-only changes