Self-host onboarding dead-ends: 'I already have a community' → 'I own the community' only offers Builderlab, never a relay URL

## Summary

An operator who runs their own relay and owns it picks the two semantically correct answers in onboarding and lands in a hosted-account flow with no way to enter their relay. The self-host path exists, but it is behind **Join a community**, which reads like the option for someone joining *somebody else's* workspace.

## Steps to reproduce

1. Stand up a self-hosted relay (auth + membership required), add your own pubkey via `buzz-admin add-member`
2. Desktop app → **Use an existing key** with the owner identity
3. **I already have a community** → **I own the community**

## Actual

A modal appears: *Set up your community — Sign in to connect a community you already own or create a new one. We'll open Builderlab in your browser, then bring you back to Buzz.* The only action is **Sign in to continue**, which opens Builderlab and creates a hosted account. At no point does the flow ask for a relay URL, and dismissing the modal returns to the same fork.

## Expected

The path that says *I own the community* should be the path where a self-hoster types their relay URL. At minimum, an escape hatch on that modal (`Connect a self-hosted relay instead`).

## What actually works

**Join a community** → paste `wss://relay.example.com` → the app connects, resolves the community, and proceeds to profile setup normally. That is the correct flow for a self-hoster; it is just labelled as if it were not.

## Impact

This is the first five minutes of the product for exactly the audience the OSS repo is aimed at. In our case it cost about forty minutes and produced a stray hosted account before we found the right door. Two smaller things compound it: the relay-owner identity is minted with `buzz-admin generate-key` in hex, which the key field rejects (#2815), and there is no visible relay-URL field until you take the Join branch.

## Suggested fix

Reword the fork, or have **I own the community** offer both `Connect my relay` and `Use Builderlab hosting`. Happy to send a PR if you want the former.