fix: prevent playback marker going back in time

Closes #141

js/src/hooks/use-stream-playback.ts

@@ -178,12 +178,22 @@ export function useStreamPlayback(
     return () => clearTimeout(timeout);
   }, [children, streamParticle.id, streamParticle.playback_markers, userId, state.initialized]);
 
-  // --- Persist playback marker ---
+  // --- Persist playback marker (only advance forward, never backwards) ---
+  const lastPersistedMarkerRef = useRef<Date | null>(null);
+
   useEffect(() => {
     if (!userId || !state.initialized || !currentParticle) return;
 
+    const currentTime = currentParticle.created_at;
+    const existingMarker =
+      lastPersistedMarkerRef.current ?? streamParticle.playback_markers?.[userId];
+
+    // Only update if advancing beyond the current marker
+    if (existingMarker && currentTime.getTime() <= existingMarker.getTime()) return;
+
+    lastPersistedMarkerRef.current = currentTime;
     const streamDocPath = toFirestoreDocPath(path);
-    updateStreamPlaybackMarker(streamDocPath, userId, currentParticle.created_at);
+    updateStreamPlaybackMarker(streamDocPath, userId, currentTime);
   }, [currentParticle?.id, state.initialized, userId, path]);
 
   // --- Navigation callbacks ---