Show member avatars on the Shared Home Display

[itsDNNS]

Summary

• Adds validated member avatar images to the Shared Home Display member payload
• Renders configured avatars on the display while keeping initials as the fallback
• Keeps the display payload narrow and strips invalid legacy avatar data

Test Plan

• DATABASE_URL=sqlite:///./test_display_devices.db JWT_SECRET=test-secret PYTHONPATH=$PWD .venv/bin/python -m pytest -q tests/test_display_devices.py tests/test_avatar_validation.py
• npm test -- --runTestsByPath __tests__/components/DisplayDashboard.test.js __tests__/pages/DisplayPage.test.js __tests__/pages/DisplayAppWrapper.test.js --runInBand
• npm test -- --runInBand
• npm run build
• npx playwright test e2e/tests/display.spec.js --project="Desktop Chrome" --project="Mobile Chrome" --reporter=list

Closes #301