test(#147): add axe-core accessibility spec with color-contrast enabled

Introduces the wcag2a/wcag2aa E2E suite from the test-suite branch with
the color-contrast rule active — no disableRules exclusion. Also adds
/coverage/ to .prettierignore so generated lcov reports don't fail the
lint hook.

This commit intentionally fails the axe suite until the contrast fixes
land in the next commits.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/.prettierignore

@@ -16,3 +16,4 @@ bun.lockb
 # Test artifacts
 /test-results/
 /e2e/.auth/
+/coverage/