fix: DM avatar color and reactions in federation + explore/server discovery

- Fix DM welcome header avatar using home identity for correct gradient color
- Register DM channel IDs in channelOriginMap so federated DM operations
  (reactions, messages, typing) route to the correct instance
- Pass origin when creating DM channels from friends list and WS events
- Add server discovery/explore page with public server listings
- Add server visibility and description fields

packages/server/src/ws/handler.ts

@@ -677,6 +677,8 @@ function buildReadyPayload(userId: string): {
         icon: serverRow.icon,
         ownerId: serverRow.ownerId,
         inviteCode: serverRow.inviteCode,
+        visibility: (serverRow.visibility ?? 'private') as ServerWithChannelsAndMembers['visibility'],
+        description: serverRow.description ?? null,
         createdAt: serverRow.createdAt,
         channels: visibleChannels,
         members,