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/stores/settingsStore.ts

@@ -18,6 +18,7 @@ const DEFAULT_LIMITS: InstanceStreamingLimits = {
   allowedFramerates: [30, 45, 60],
   maxResolution: 1080,
   maxFramerate: 60,
+  discoveryEnabled: true,
 };
 
 export function getStreamingLimits(): InstanceStreamingLimits {