Desktop App stuck on Builderlab onboarding — cannot use with local relay

## Description

The Desktop App forces users through the Builderlab onboarding flow to create a community. When Builderlab is unreachable (e.g., Cloudflare block, network restrictions, or self-hosted relay setup), the app shows a full-screen "Set up your community" page with no way to skip, close, or bypass it.

## Steps to Reproduce

1. Download `Buzz_0.5.20_amd64.AppImage`
2. Start a local relay via Docker/Podman (`ghcr.io/block/buzz:main`)
3. Set `relay_url = "ws://localhost:3000"` in `~/.config/xyz.block.buzz.app/config.toml`
4. Launch the Desktop App
5. App shows "Set up your community" with "Waiting for your browser..."
6. Builderlab is unreachable → user is permanently stuck on this screen

## Expected Behavior

The Desktop App should allow users to use a local relay without requiring Builderlab authentication. Options could include:
- A "Skip" or "Use local relay" button on the onboarding screen
- Auto-detecting an existing local relay and bypassing the Builderlab flow
- A CLI flag or config option to skip the Builderlab onboarding

## Actual Behavior

The full-screen "Set up your community" page has:
- No close button
- No skip option
- No way to proceed without Builderlab
- The app is completely unusable

## Environment

- **OS:** Fedora Linux (x86_64)
- **AppImage:** Buzz_0.5.20_amd64.AppImage
- **Local Relay:** ghcr.io/block/buzz:main via Podman
- **Browser:** Zen Browser (Flatpak)

## Context

The `buzz` CLI works perfectly with the local relay. The Desktop App connects to the relay and sends events (confirmed via relay logs), but the frontend is blocked on the Builderlab screen.

This makes the Desktop App unusable for anyone running a self-hosted relay without access to Builderlab.

Start with the Desktop App's "Set up your community" onboarding entry point and the config.toml relay_url setting described in the report. Compare how the CLI reaches a local relay while Builderlab is unavailable, then trace why the Desktop App remains blocked. Done means a configured local relay can be used without Builderlab, with a clear bypass path for the blocked onboarding state.

Written by the indexing model from the issue text.