Mobile: no display-name / kind-0 profile editor — mobile-created identities appear as raw hex to everyone

## Summary

The mobile app reads kind-0 profiles for rendering (avatars, mention pickers) but provides **no way to set or edit your own display name** — Settings offers only a status field. Any identity created on mobile (invite join, `buzz://` payload) is permanently unnamed and appears as a raw hex prefix (`e1e0d007…`) in the @-mention picker and member lists, for everyone in the community.

## Impact

- Mobile-only users (the exact audience of #2735 / #2662) can never name themselves; communities fill up with hex identities that other members can't tell apart.
- Desktop can edit profiles, so the workaround today is "borrow a desktop once" or have an operator publish a kind-0 for you server-side with your key — neither is viable for normal mobile-first onboarding (family members, field employees).

## Reproduction

1. Join a community on iOS via an invite (fresh identity generated on device).
2. Open the @-mention picker in any channel → your identity renders as a hex prefix.
3. Look for any name/profile editor in Settings → only "status" exists.

## Suggestion

A display-name field in mobile onboarding (at join time) plus an editor in Settings that publishes the kind-0. Even a name-only editor (no avatar) would remove the hex-identity problem for mobile-first rollouts.