Test: wire jest-axe + apply a11y assertions to component tests

[wab]

Parent

#121

What to build

Wire the jest-axe matcher in the test setup (if not already done in P0) and add a11y assertions to existing component tests. If P0 already shipped this, this issue becomes a verification ticket — confirm and close.

Acceptance criteria

• jest-axe matcher registered in app/test/setup.ts
• toHaveNoViolations assertion available globally
• At least 3 existing component tests assert no a11y violations
• If P0 already wired this, close with a confirmation comment + screenshot of green run

Blocked by

None - can start immediately