a11y(frontend): enhance accessibility support - Sprint 3 Story 5

Improve accessibility to meet WCAG 2.1 Level AA standards.

Changes: Added eslint-plugin-jsx-a11y, keyboard navigation, ARIA labels, SkipLink component, main-content landmark.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/ui/skip-link.tsx

@@ -0,0 +1,10 @@
+export function SkipLink() {
+  return (
+    <a
+      href="#main-content"
+      className="sr-only focus:not-sr-only focus:absolute focus:top-4 focus:left-4 focus:z-50 focus:px-4 focus:py-2 focus:bg-primary focus:text-primary-foreground focus:rounded-md focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2"
+    >
+      Skip to main content
+    </a>
+  );
+}