Adopt a WCAG 2.1 AA accessibility gate

**Context**

The angular-upgrade-dashboard's quality metrics table (https://gsa.github.io/angular-upgrade-dashboard/#metrics) reports an accessibility gate for `sam-styles` and `ngx-uswds`, both of which run `playwright test --config=playwright.a11y.config.{mjs,ts}` (`npm run test:a11y`) as a CI gate enforcing WCAG 2.1 AA. sam-ui-elements has no equivalent — only a Playwright smoke test (`e2e.yml`) with no accessibility assertions — so the dashboard reports `not enforced`.

**Ask**

Add an axe-core-backed a11y gate against the Storybook/test-app stories, mirroring the `playwright.a11y.config` approach already proven in `sam-styles` and `ngx-uswds`. Given sam-ui-elements is the most actively upgraded library in this epic, closing this gap here has the highest near-term value of the four repos.

Filed as a sub-issue of the pipeline modernization epic; found while building the dashboard's quality metrics grid.

Start by inspecting the existing e2e.yml and the Storybook/test-app setup, then compare the playwright.a11y.config.{mjs,ts} and npm run test:a11y patterns in sam-styles and ngx-uswds. Add the axe-core-backed gate for the stories and verify that CI enforces WCAG 2.1 AA rather than only smoke coverage.

Written by the indexing model from the issue text.