feat(a11y): add jest-axe and jsx-a11y with baseline for SettlementTable and MetricsBar

[Opulencechuks]

Closes #428

Description

This PR introduces accessibility verification to the AnchorNet-Frontend codebase targeting the WCAG 2.1 AA baseline. To prevent a11y regressions without overwhelming the team with existing violations on day one, I have implemented a targeted baseline approach.

Layer-Ranking & Scope Strategy:

• jest-axe (Highest value-per-effort): Leverages our existing vitest suite to catch rendered-output violations without new infrastructure. I have scoped this gate to our highest-traffic and most data-dense components (SettlementTable and MetricsBar).
• eslint-plugin-jsx-a11y (High value-per-effort): Added to the flat config to catch static markup problems cheaply at author time directly in the editor.

Changes Made:

• Configured eslint-plugin-jsx-a11y and jest-axe.
• Fixed existing a11y violations in SettlementTable (added scope="col" to headers and sr-only text to action columns).
• Baselined ToastProvider by suppressing a tabIndex warning with a documented reason, keeping its keyboard operability intact while keeping the build green.
• Added a11y.test.tsx which asserts zero violations for SettlementTable and MetricsBar, and verifies keyboard operability for actionable settlements.

Checklist

• I added a CHANGELOG.md entry under the next ## [x.y.z] section
  (see the Format note at the top of CHANGELOG.md), or this PR
  is docs-only / test-only / internal tooling and doesn't change
  user-facing behavior.
• Tests added/updated for the change.