feat: show latest particle preview in stream list

js/src/lib/firestore-particles.ts

@@ -7,6 +7,7 @@ import {
   updateDoc,
   query,
   orderBy,
+  limit,
   serverTimestamp,
   Timestamp,
   type DocumentData,
@@ -97,6 +98,25 @@ export function subscribeToParticleChildren(
   );
 }
 
+export function subscribeToLatestChild(
+  collectionPath: string,
+  onData: (child: Particle | null) => void,
+  onError: (error: Error) => void,
+): Unsubscribe {
+  const q = query(
+    typedCollection(collectionPath),
+    orderBy("created_at", "desc"),
+    limit(1),
+  );
+  return onSnapshot(
+    q,
+    (snap) => {
+      onData(snap.empty ? null : snap.docs[0].data());
+    },
+    onError,
+  );
+}
+
 // This creates a new particle document with the given properties and returns its ID.
 export async function createParticle<T extends ParticleType>(
   collectionPath: string,