test(global-modal): add comprehensive unit tests for GlobalModal (Vitest + RTL)

[VetteIT]

Description

Adds a comprehensive unit test suite for the GlobalModal component using Vitest + React Testing Library.
Covers portal rendering to #modal-container (with fallback to document.body), state-driven modal selection, and rerender behavior.

Closes #112

---

What change does this PR introduce?

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

---

What is the proposed approach?

• Mocked useGlobalModal via a hoisted state to avoid mocking order issues and enable clear assertions.
• Stubbed LoginModal and ErrorModal with lightweight test doubles (data-testid hooks).
• Verified portal targets by DOM queries (container vs. body) following RTL best practices.
• Asserted priority behavior (error > login) and rerender transition (login → error).

---

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)