Serialize file saves so an older write can't overwrite a newer one

Addresses review feedback: with autosave, a second save could start
while the first was still writing. If the older write finished last it
overwrote the newer content on disk with nothing left marked unsaved.

handleFileSave now queues saves and runs them one at a time; each queued
save reads the current content when it starts. A failed save doesn't
block later ones.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: NGdev2

frontend/app/view/preview/preview-model.tsx

@@ -140,6 +140,7 @@ export class PreviewModel implements ViewModel {
     previewTextRef: React.RefObject<HTMLDivElement>;
     editMode: Atom<boolean>;
     autoSaveMode: Atom<AutoSaveMode>;
+    saveQueue: Promise<void> = Promise.resolve();
     canPreview: PrimitiveAtom<boolean>;
     specializedView: Atom<Promise<{ specializedView?: string; errorStr?: string }>>;
     loadableSpecializedView: Atom<Loadable<{ specializedView?: string; errorStr?: string }>>;
@@ -658,7 +659,16 @@ export class PreviewModel implements ViewModel {
         await this.env.services.object.UpdateObjectMeta(blockOref, { ...blockMeta, edit });
     }
 
-    async handleFileSave() {
+    handleFileSave(): Promise<void> {
+        // run saves one at a time; a queued save reads newFileContent when it starts, so an
+        // older write can never finish after (and overwrite) a newer one
+        this.saveQueue = this.saveQueue
+            .then(() => this.writeFileContent())
+            .catch((e) => console.log("error saving file", e));
+        return this.saveQueue;
+    }
+
+    async writeFileContent() {
         const filePath = await globalStore.get(this.statFilePath);
         if (filePath == null) {
             return;