test(modal): add comprehensive unit tests for LoginModal (Vitest + RTL)

[VetteIT]

Description

Adds a comprehensive unit test suite for the LoginModal component using Vitest + React Testing Library.
Covers rendering, i18n labels, login button behavior, close interactions, and Modal wiring (open, aria-labelledby, onClose).

Closes #114

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What change does this PR introduce?

• Bug fix
• New feature
• Breaking change
• CI/CD
• Documentation update
• Chore (tests)

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What is the proposed approach?

• Mocked MUI Modal to capture and assert open, aria-labelledby, and invoke onClose.
• Mocked useGlobalModal to verify setPromptLogin(false) on both close icon click and Modal onClose.
• Mocked handleLogin and asserted it fires when the GitHub login button is clicked.
• i18n mock returns keys for deterministic assertions.

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Checklist:

• The commit message follows our adopted guidelines
• Testing has been done for the change(s) added
• Relevant comments/docs have been added/updated (N/A — tests only)