Add Libravatar contact avatar lookup

[keilogic]

Summary

• add Libravatar as a fallback after Gravatar for remote contact avatar lookups
• keep local contact photos preferred, and cache a missing avatar only after both remote services miss
• update the Contacts avatar setting text and cover Gravatar, Libravatar, and no-avatar lookup behavior in the Contacts service spec

Testing

• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• npx ng test --include src/app/contacts-app/contacts.service.spec.ts --watch=false --browsers=FirefoxHeadless
• npm run policy (passes; prints existing historical upstream commit-message warnings)
• SKIP_CHANGELOG=1 node src/build/pre-build.js
• npx ng build runbox7 --configuration production --base-href=/app/
• node src/build/post-build.js

AI disclosure

I used OpenAI Codex to implement and test this change in:

• src/app/contacts-app/contacts.service.ts
• src/app/contacts-app/contacts.service.spec.ts
• src/app/contacts-app/contacts-settings.component.html

I reviewed the generated diff and ran the validation steps above.

Fixes #1177