fix(dm): render system messages in sidebar preview instead of raw JSON

DmLastMessagePreview lacked a `type` field, so the sidebar rendered
`lastMessage.content` verbatim — surfacing JSON like
`{"event":"space_invite",...}` for space invites and member-add events.

Adds `type` to the preview payload (populated server-side from
`dm_messages.type`) and routes all sidebar call sites through a single
`formatDmSidebarPreview` helper that renders human-readable text for
each system event and skips the group `Sender:` prefix on system rows.

packages/shared/src/types.ts

@@ -294,6 +294,12 @@ export interface DmLastMessagePreview {
   userId: string;
   content: string | null;
   createdAt: number;
+  /**
+   * 'system' for membership/lifecycle JSON payloads (member_added, member_removed,
+   * owner_changed, space_invite). 'user' (or omitted) for normal user-authored
+   * messages. The sidebar renderer relies on this to avoid showing raw JSON.
+   */
+  type?: 'user' | 'system';
   attachments?: Array<{ type: string; filename: string }>;
 }