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/web/src/components/chat/FriendsPage.tsx

@@ -59,7 +59,7 @@ export function FriendsPage() {
         if (instance?.api) client = instance.api;
       }
       const dmChannel = await client.dm.create({ userId: friendId });
-      addDmChannel(dmChannel);
+      addDmChannel(dmChannel, instanceOrigin);
       navigate(`/channels/@me/${dmChannel.id}`);
     } catch (err) {
       console.error('Failed to open DM:', err);