feat: wire buildSources into loadServerSettings startup

After fetching /api/settings and caching in _serverSettings,
call _sources = buildSources(_serverSettings) so the polling
layer immediately knows which sources to query.

Implements task-6 of multi-device federation phase 1.

muxplex/frontend/app.js

@@ -1027,6 +1027,7 @@ async function loadServerSettings() {
     console.warn('[loadServerSettings] failed:', err);
     if (!_serverSettings) _serverSettings = {};
   }
+  _sources = buildSources(_serverSettings);
   return _serverSettings;
 }