Add bloglist social metadata and favicon

[mashalink]

What changed

• added Open Graph and Twitter meta tags for the Bloglist frontend
• replaced the default Vite favicon with a project-specific favicon
• added social preview assets for sharing (preview.svg and preview.png)
• added the production app URL to the backend README

Why

• improve how Bloglist looks when shared in social platforms and messengers
• make the project branding consistent in the browser tab and previews
• make the deployed app URL easier to find in the documentation

Validation

• ran npm run build in part7/bloglist-frontend-redux
• verified that preview.png was regenerated correctly at 1200x630
• confirmed the branch was pushed successfully to origin