feat: reconcile when saving to firebase (#4991)

* naming tweaks

* do not mark local element as duplicate when there's no remote counterpart

* merge instead of overwrite elements when saving to firebase & reconcile local state

* decouple syncing from persistence

* fix ts

* clarify doc

* fix reconciliation not removing duplicates

src/excalidraw-app/index.tsx

@@ -455,7 +455,7 @@ const ExcalidrawWrapper = () => {
     files: BinaryFiles,
   ) => {
     if (collabAPI?.isCollaborating()) {
-      collabAPI.broadcastElements(elements);
+      collabAPI.syncElements(elements);
     }
 
     // this check is redundant, but since this is a hot path, it's best