fix: prevent restart playback after transcoding completion

Closes #201

js/desktop/src/features/particles/media-particle-view.tsx

@@ -35,10 +35,20 @@ export const MediaParticleView = forwardRef<MediaParticleHandle, MediaParticleVi
 }, ref) {
   // Prefer the worker-produced iOS-playable variant when present so desktop
   // and mobile read the same canonical asset. Falls back to the original.
-  const activeObjectId =
-    particle.properties.transcoded_object_id ?? particle.properties.object_id;
-  const activeMime =
-    particle.properties.transcoded_mime_type ?? particle.properties.mime_type;
+  // Pin the choice for the lifetime of this particle: if a transcoded variant
+  // arrives via Firestore mid-playback, swapping the <video> src would restart
+  // playback from 0. Keep whatever we picked first; the original plays fine in
+  // Electron, and the transcoded variant will be picked up on the next view.
+  const pickedSourceRef = useRef<{ id: string; objectId: string; mime: string } | null>(null);
+  if (pickedSourceRef.current?.id !== particle.id) {
+    pickedSourceRef.current = {
+      id: particle.id,
+      objectId: particle.properties.transcoded_object_id ?? particle.properties.object_id,
+      mime: particle.properties.transcoded_mime_type ?? particle.properties.mime_type,
+    };
+  }
+  const activeObjectId = pickedSourceRef.current.objectId;
+  const activeMime = pickedSourceRef.current.mime;
   const { data: url, error } = useDownloadUrl(activeObjectId);
   const { attachments } = useParticleAttachments(streamPath, particle.id);