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>

package.json

@@ -59,6 +59,7 @@
     "@types/react-dom": "^19",
     "eslint": "^9",
     "eslint-config-next": "16.0.1",
+    "eslint-plugin-jsx-a11y": "^6.10.2",
     "prettier": "^3.6.2",
     "prettier-plugin-tailwindcss": "^0.7.1",
     "tailwindcss": "^4",