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/server/src/ws/handler.ts

@@ -1328,6 +1328,7 @@ function buildReadyPayload(userId: string): {
           userId: last.userId,
           content: last.content,
           createdAt: last.createdAt,
+          type: last.type === 'system' ? 'system' : 'user',
           attachments: dmLastMsgAttachmentMap.get(last.id) ?? [],
         } : null,
       });