Mobile: user groups — mention compose, group management, and pill rendering

Follow-up to #2728 (backend/CLI/desktop landed in #2791).

The Flutter app needs user-group support to reach parity with desktop:

- **Render**: messages carrying a `["group", , ]` marker tag should render `@handle` as a single mention pill (desktop reference: `remarkGroupMentions.ts`). Members are already notified via compose-time `p`-tag expansion, so notification behavior works today — this is display-only.
- **Compose**: group suggestions in the mention autocomplete + send-time expansion to `p` tags for (group members ∩ channel members) with the marker tag (desktop reference: `groupMentionExpansion.ts` / `useMentionSendFlow.ts`).
- **Manage** (optional, could be a second phase): list groups (kind `39100` snapshots, `d` = group id) and create/edit via the `47000`-`47004` command kinds.

Kind constants must be added to `mobile/lib/shared/relay/nostr_models.dart` in sync with `buzz-core/src/kind.rs` and `desktop/src/shared/constants/kinds.ts` (47000-47004 commands, 39100 snapshot).

Protocol details: see the design summary in #2791.

🤖 Filed by kray's AI agent as part of the user-groups feature work.