test(ui): add jest-axe accessibility checks

[MerlinTheWhiz]

Closes #67

Changes Made:

• package.json (Modified): Added jest-axe dev dependency
• jest.setup.js (Modified): Registered toHaveNoViolations matcher from jest-axe
• src/test-utils/a11y.tsx (New): Shared helpers: testA11y(), assertNoA11yViolations(), renderWithA11y()
• src/components/tests/a11y.test.tsx (New): 15 axe tests across 4 components
• docs/components/Accessibility.md (New): Documents setup, usage, and CI integration

Test Coverage (all pass, zero violations):

• MilestonesList — empty, single, multiple statuses, missing dueDate
• ContractSummary — Active+multiple parties, Disputed, Completed+solo milestone
• ReputationProfile — no reputation, full score+history, partial (score only), null score
• EmptyState — text-only, illustration variant, primary action, both actions

How it works:

// One-liner in tests:
await testA11y(<Component prop="value" />);

The testA11y helper renders the component, runs axe() on the container, and asserts toHaveNoViolations(). Axe defaults to WCAG 2.1 AA rules — no custom config needed since components already use proper ARIA landmarks, labels, roles, focus rings, and color contrast.

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[MerlinTheWhiz]

@mikewheeleer Hey boss, this pr is still up. Please review and merge