Fix test assertions and mock cleanup per code review

[Copilot]

Addresses code review feedback on test quality and configuration safety from PR #26.

Test Assertions

Replaced toBeDefined() with not.toBeNull() for querySelector() results (12 instances across 3 files). toBeDefined() passes for null, creating false positives:

// Before: passes even when modal doesn't exist
const modal = document.querySelector('.modal.show');
expect(modal).toBeDefined();  // ✗ null is defined

// After: correctly fails when element missing
expect(modal).not.toBeNull();  // ✓ fails on null

Mock Hygiene

Saved and restored window.open in 4 errorBoundary.test.jsx tests to prevent leakage between test runs.

Code Cleanup

• Removed .vscode/settings.json (auto-approved git push/git commit - unsafe for contributors/CI)
• Removed unused NavigatableRights helper and imports from rights.test.jsx
• Removed tautological test checking for non-existent modal element

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