fix: give Explore its own /explore route so PiP navigates back to voice correctly

The Explore page shared /channels/@me with Friends, differentiated by a
showExplore UI flag. When PiP navigated to a voice channel, the stale
flag caused MainContent to render text chat instead of the voice grid.
Replace the flag with a dedicated /explore route so the URL is the
single source of truth.

packages/web/src/components/layout/AppLayout.tsx

@@ -183,14 +183,15 @@ export function AppLayout() {
   // Handle route params
   useEffect(() => {
     if (serverId === '@me') {
-      if (!useUIStore.getState().showExplore) {
-        setShowDms(true);
-      }
+      setShowDms(true);
       setCurrentServer(null);
     } else if (serverId) {
       setShowDms(false);
       setCurrentServer(serverId);
       loadServerDetail(serverId);
+    } else {
+      setShowDms(false);
+      setCurrentServer(null);
     }
   }, [serverId, setCurrentServer, loadServerDetail, setShowDms]);