Second-community join flow reads as creating a new identity; prefill from the existing profile

## Summary

The desktop holds exactly one identity (`identity.key`); `Community.pubkey` is display-only. Joining a **second** community nevertheless opens a build-a-profile-from-scratch flow whose wording ("create your avatar") reads as minting a **new identity**. Users who deliberately run one key across communities hesitate here, and the only reassurance is indirect: after joining, a private channel you were already a member of shows up — proving the key is the same.

The relay is right to refuse copying a `kind:0` across tenants — that isolation is the point. The client is the part that could help: it holds the single `identity.key`, it has both communities in its own list, and it could prefill name/avatar from the profile it already has and say plainly that the identity is unchanged.

## Environment

- Buzz Desktop **0.5.8**, macOS (Apple Silicon)
- Self-hosted relay, two host-keyed communities, one owner key

## Expected

- Prefill the second community's profile form from an existing community's `kind:0` (the user stays free to diverge per community).
- One line of copy stating that the identity/npub stays the same across communities.

Start in the desktop client’s second-community join flow and trace how the existing identity.key and community kind:0 profile reach the profile form. The work is done when the form is prefilled from an existing community profile, users can diverge from it, and the UI states that the identity/npub remains unchanged across communities.

Written by the indexing model from the issue text.