fix(desktop): improve sidebar reactivity for new DMs and channel updates

Wire live mention WebSocket events to also invalidate the channels query,
so the sidebar refreshes immediately when activity is detected. Reduce
channels polling interval from 60s to 15s as a safety net for genuinely
new channels (e.g. bot-created DMs) that the WebSocket subscription
doesn't know about yet.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/app/AppShell.tsx

@@ -155,6 +155,7 @@ export function AppShell() {
     );
   const refetchHomeFeedOnLiveMention = React.useEffectEvent(() => {
     void homeFeedQuery.refetch();
+    void queryClient.invalidateQueries({ queryKey: channelsQueryKey });
   });
 
   const handleDmNotification = React.useEffectEvent(

desktop/src/features/channels/hooks.ts

@@ -109,8 +109,8 @@ export function useChannelsQuery() {
   return useQuery({
     queryKey: channelsQueryKey,
     queryFn: async () => sortChannels(await getChannels()),
-    staleTime: 60_000,
-    refetchInterval: 60_000,
+    staleTime: 15_000,
+    refetchInterval: 15_000,
     refetchIntervalInBackground: false,
   });
 }