Polish community profile onboarding UI

[cynfria]

Why

Polish the first-community profile step without changing onboarding behavior or broadening scope.

What

• Present the avatar as the visual focus of the profile step
• Replace the oversized key-style name row with a standard labeled display-name field
• Preserve the existing display-name semantics, onboarding lifecycle, navigation, avatar editor, and camera behavior
• Update the focused E2E visual assertions

Scope

UI-only: one onboarding component and its E2E assertion. No relay, app-state, navigation, persistence, camera, settings, shared-style, or avatar-editor-internal changes.

Validation

• Desktop build
• Desktop typecheck and lint
• Focused onboarding integration tests
• Full pre-push suite

[wesbillman]

Let's get the demo stuff clean up here

[wesbillman]

Requesting changes because this is not a UI-only change and it diverges from the current onboarding contract.

1. Non-UI production behavior is bundled into the PR. crates/buzz-pair-relay/src/lib.rs changes the relay's production connection-limit implementation and exposes a new constructor; desktop/src/app/App.tsx changes first-community cancellation/back behavior by clearing communities and returning to Welcome; AnimatedAvatarCapture.tsx now auto-starts the camera in compact mode. Those are runtime behavior changes, not presentation. Please remove them from this PR (and split any independently needed fixes).

2. The profile field is presented as a username but persisted as the display name. In CommunityOnboardingFlow.tsx, the UI says “Your username” and suggests @yourusername, but saveProfile() writes that value to displayName. Current main asks for “First and last name,” matching the profile/display-name model. This would teach a different concept while storing it in the existing display-name field. Please keep the current display-name semantics/copy unless a real username model is being introduced separately.

The branch is also 4 commits behind current main, including recent onboarding/startup work, and overlaps many of the same files. Once the PR is reduced to styling/layout-only changes and aligned with current main, CI should run on that resulting head.