Feature Request: Custom display name & avatar for the profile blog

### What

Hi! Currently, the ActivityPub blog profile gets its display name (the public name of the federated account as seen in the Fediverse) from the Site title. It would be great to have a specific field for that. Same applies for the site icon / avatar.
Thanks!

### Why

The site title may not always match the desired name for the federated account feed, i.e. "Lorem ipsum - Travel Agency" vs "Free city guides" or "Jane Doe · Photographer in New York" vs. "NY images by JD". Sometimes the blog content needs its own name or brand & icon.

### How

Just like the description field works: by default it uses the site tagline but accepts a custom description.

No files or tests are named. Start by locating the profile blog settings and the existing handling for the site tagline and icon, then trace how those values reach the ActivityPub profile; done means custom display name and avatar fields work while preserving the current site title and icon defaults.

Written by the indexing model from the issue text.